clean up E0392 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0392.md

@@ -1,5 +1,6 @@
-This error indicates that a type or lifetime parameter has been declared
-but not actually used. Here is an example that demonstrates the error:
+A type or lifetime parameter has been declared but is not actually used.
+
+Erroneous code example:
 
 ```compile_fail,E0392
 enum Foo<T> {