adds needed apostrophe

README.md

@@ -166,7 +166,7 @@ In the example code for App, you may have noticed something odd:
   }
 ```
 
-We have a prop named dispatch! But where did it come from if its a prop? We 
+We have a prop named dispatch! But where did it come from if it's a prop? We 
 will go into greater detail later, but `dispatch` is automatically provided
 by `connect` if it is missing a _second_ argument. That second argument is
 reserved for `mapDispatchToProps`, which allows us to customize how we send