README.md: Correcting punctuation

Author: Andrew Crerar

README.md

@@ -13,7 +13,7 @@ You can follow me on Twitter [@SamyPesse](https://twitter.com/SamyPesse) or supp
 
 ### What kind of OS are we building?
 
-The goal is to build a very simple UNIX-based operating system in C++, not just a "proof-of-concept". The OS should be able to boot, start an userland shell and be extensible.
+The goal is to build a very simple UNIX-based operating system in C++, not just a "proof-of-concept". The OS should be able to boot, start a userland shell, and be extensible.
 
 [![Screen](https://raw.github.com/SamyPesse/How-to-Make-a-Computer-Operating-System/master/preview.png)](https://raw.github.com/SamyPesse/How-to-Make-a-Computer-Operating-System/master/preview.png)