Update README.md

Author: norvig

README.md

@@ -35,7 +35,7 @@ For each notebook you can:
 
 |Run|Year|Programming Examples|
 |---|---|---|
-| [c](https://colab.research.google.com/github/norvig/pytudes/blob/main/ipynb/Advent-2020.ipynb) [d](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmain%2Fipynb%2FAdvent-2020.ipynb)  [m](https://mybinder.org/v2/gh/norvig/pytudes/main?filepath=ipynb%2FAdvent-2020.ipynb) [s](https://studiolab.sagemaker.aws/import/github/norvig/pytudes/blob/main/ipynb/Advent-2020.ipynb) [n](https://nbviewer.jupyter.org/github/norvig/pytudes/blob/main/ipynb/Advent-2020.ipynb) | 2021 | <b><a href="ipynb/Advent-2020.ipynb" title="Puzzle site with a coding puzzle each day for Advent 2021">Advent of Code 2021</a></b> |
+| [c](https://colab.research.google.com/github/norvig/pytudes/blob/main/ipynb/Advent-2021.ipynb) [d](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmain%2Fipynb%2FAdvent-2021.ipynb)  [m](https://mybinder.org/v2/gh/norvig/pytudes/main?filepath=ipynb%2FAdvent-2021.ipynb) [s](https://studiolab.sagemaker.aws/import/github/norvig/pytudes/blob/main/ipynb/Advent-2021.ipynb) [n](https://nbviewer.jupyter.org/github/norvig/pytudes/blob/main/ipynb/Advent-2021.ipynb) | 2021 | <b><a href="ipynb/Advent-2021.ipynb" title="Puzzle site with a coding puzzle each day for Advent 2021">Advent of Code 2021</a></b> |
 | [c](https://colab.research.google.com/github/norvig/pytudes/blob/main/ipynb/Advent-2020.ipynb) [d](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmain%2Fipynb%2FAdvent-2020.ipynb)  [m](https://mybinder.org/v2/gh/norvig/pytudes/main?filepath=ipynb%2FAdvent-2020.ipynb) [s](https://studiolab.sagemaker.aws/import/github/norvig/pytudes/blob/main/ipynb/Advent-2020.ipynb) [n](https://nbviewer.jupyter.org/github/norvig/pytudes/blob/main/ipynb/Advent-2020.ipynb) | <u>2020</u> | <b><a href="ipynb/Advent-2020.ipynb" title="Puzzle site with a coding puzzle each day for Advent 2020">Advent of Code 2020</a></b> |
 | [c](https://colab.research.google.com/github/norvig/pytudes/blob/main/ipynb/Advent-2018.ipynb) [d](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmain%2Fipynb%2FAdvent-2018.ipynb)  [m](https://mybinder.org/v2/gh/norvig/pytudes/main?filepath=ipynb%2FAdvent-2018.ipynb) [s](https://studiolab.sagemaker.aws/import/github/norvig/pytudes/blob/main/ipynb/Advent-2018.ipynb) [n](https://nbviewer.jupyter.org/github/norvig/pytudes/blob/main/ipynb/Advent-2018.ipynb) | 2018 | <b><a href="ipynb/Advent-2018.ipynb" title="Puzzle site with a coding puzzle each day for Advent 2018">Advent of Code 2018</a></b> |
 | [c](https://colab.research.google.com/github/norvig/pytudes/blob/main/ipynb/Advent%202017.ipynb) [d](https://beta.deepnote.org/launch?template=python_3.6&url=https%3A%2F%2Fgithub.com%2Fnorvig%2Fpytudes%2Fblob%2Fmain%2Fipynb%2FAdvent%202017.ipynb)  [m](https://mybinder.org/v2/gh/norvig/pytudes/main?filepath=ipynb%2FAdvent%202017.ipynb) [s](https://studiolab.sagemaker.aws/import/github/norvig/pytudes/blob/main/ipynb/Advent%202017.ipynb) [n](https://nbviewer.jupyter.org/github/norvig/pytudes/blob/main/ipynb/Advent%202017.ipynb) | 2017 | <b><a href="ipynb/Advent%202017.ipynb" title="Puzzle site with a coding puzzle each day for Advent 2017">Advent of Code 2017</a></b> |