Fix typo

Author: zohairawan

src/interfaces.md

@@ -2,7 +2,7 @@
 
 Generics let you write code that doesn't care
 what is different between different things - you would accept a `String`
-or an `Integer`, doesn't matter what is diferent between them.
+or an `Integer`, doesn't matter what is different between them.
 
 Interfaces do a related thing. They let you write code that takes advantage
 of commonalities.