[Practice Exercise Documentation]: Update add.md (#573)

Added the word "add" to line 3.

Author: BethanyG

building/tracks/practice-exercises/add.md

@@ -1,6 +1,6 @@
 # Add Practice Exercise
 
-This document will explain how to a new [Practice Exercise](/docs/building/tracks/practice-exercises).
+This document will explain how to add a new [Practice Exercise](/docs/building/tracks/practice-exercises).
 
 ## Select exercise