clean E0271 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0271.md

@@ -1,9 +1,6 @@
-This is because of a type mismatch between the associated type of some
-trait (e.g., `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
-and another type `U` that is required to be equal to `T::Bar`, but is not.
-Examples follow.
+A type mismatched an associated type of a trait.
 
-Here is a basic example:
+Erroneous code example:
 
 ```compile_fail,E0271
 trait Trait { type AssociatedType; }
@@ -17,6 +14,11 @@ impl Trait for i8 { type AssociatedType = &'static str; }
 foo(3_i8);
 ```
 
+This is because of a type mismatch between the associated type of some
+trait (e.g., `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
+and another type `U` that is required to be equal to `T::Bar`, but is not.
+Examples follow.
+
 Here is that same example again, with some explanatory comments:
 
 ```compile_fail,E0271