Update README.md

Author: Chris Owen

README.md

@@ -1,6 +1,6 @@
 Like learning a musical instrument, programming requires daily practise.
 
-The exercises are split into three folders: `exercises`, `mandatory` and `extra`. All homework in the `exercise and ``mandatory` section **must** be completed for homework by the following lesson. 
+The exercises are split into three folders: `exercises`, `mandatory` and `extra`. All homework in the `exercise` and `mandatory` section **must** be completed for homework by the following lesson. 
 
 The `extra` folder contains exercises that you can complete to challenge yourself, but are not required for the following lesson.