clean up E0201 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0201.md

@@ -1,7 +1,7 @@
-It is an error to define two associated items (like methods, associated types,
-associated functions, etc.) with the same identifier.
+Two associated items (like methods, associated types, associated functions,
+etc.) were defined with the same identifier.
 
-For example:
+Erroneous code example:
 
 ```compile_fail,E0201
 struct Foo(u8);