Fix bad filename for README in packaging tutorial (pypa#507)

Author: theacodes

source/tutorials/packaging-projects.rst

@@ -70,7 +70,7 @@ the values if you want:
 
     import setuptools
 
-    with open("README.rst", "r") as fh:
+    with open("README.md", "r") as fh:
         long_description = fh.read()
 
     setuptools.setup(
@@ -343,4 +343,4 @@ some things you can do:
   and `poetry`_.
 
 .. _hatch: https://github.com/ofek/hatch
-.. _poetry: https://github.com/sdispater/poetry
+.. _poetry: https://github.com/sdispater/poetry