Fix NESButtons typo

Just a little typo in the enum name.

concepts/enumerations/introduction.md

@@ -27,7 +27,7 @@ enum NESButton {
 }
 ```
 
-This defines a new type named `NESButtons` with possible values `up`, `down`, `left`, `right`, `a`, `b`, `select`, and `start`. These values can be referred to by following the name of the type with a dot (`.`) and the value. In cases where the type name can be inferred, only the dot and value are needed.
+This defines a new type named `NESButton` with possible values `up`, `down`, `left`, `right`, `a`, `b`, `select`, and `start`. These values can be referred to by following the name of the type with a dot (`.`) and the value. In cases where the type name can be inferred, only the dot and value are needed.
 
 ## Methods