clean up E0276 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0276.md

@@ -1,5 +1,6 @@
-This error occurs when a bound in an implementation of a trait does not match
-the bounds specified in the original trait. For example:
+A trait implementation has stricter requirements than the trait definition.
+
+Erroneous code example:
 
 ```compile_fail,E0276
 trait Foo {