Clean up E0437 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0437.md

@@ -1,7 +1,5 @@
-Trait implementations can only implement associated types that are members of
-the trait in question. This error indicates that you attempted to implement
-an associated type whose name does not match the name of any associated type
-in the trait.
+An associated type whose name does not match any of the associated types
+in the trait was used when implementing the trait.
 
 Erroneous code example:
 
@@ -13,6 +11,9 @@ impl Foo for i32 {
 }
 ```
 
+Trait implementations can only implement associated types that are members of
+the trait in question.
+
 The solution to this problem is to remove the extraneous associated type:
 
 ```