Clean up E0091 long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0091.md

@@ -1,5 +1,6 @@
-You gave an unnecessary type or const parameter in a type alias. Erroneous
-code example:
+An unnecessary type or const parameter was given in a type alias.
+
+Erroneous code example:
 
 ```compile_fail,E0091
 type Foo<T> = u32; // error: type parameter `T` is unused