Update README.md

Small grammar correction.

Author: justin0022

README.md

@@ -291,7 +291,7 @@ createMenu({
 
 This is by far the most important rule in software engineering. When functions
 do more than one thing, they are harder to compose, test, and reason about.
-When you can isolate a function to just one action, they can be refactored
+When you can isolate a function to just one action, it can be refactored
 easily and your code will read much cleaner. If you take nothing else away from
 this guide other than this, you'll be ahead of many developers.