Clean up E0191 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0191.md

@@ -1,5 +1,6 @@
-Trait objects need to have all associated types specified. Erroneous code
-example:
+An associated type wasn't specified for a trait object.
+
+Erroneous code example:
 
 ```compile_fail,E0191
 trait Trait {
@@ -10,8 +11,9 @@ type Foo = Trait; // error: the value of the associated type `Bar` (from
                   //        the trait `Trait`) must be specified
 ```
 
-Please verify you specified all associated types of the trait and that you
-used the right trait. Example:
+Trait objects need to have all associated types specified. Please verify that
+all associated types of the trait were specified and the correct trait was used.
+Example:
 
 ```
 trait Trait {