clean up E0321 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0321.md

@@ -1,5 +1,7 @@
 A cross-crate opt-out trait was implemented on something which wasn't a struct
-or enum type. Erroneous code example:
+or enum type.
+
+Erroneous code example:
 
 ```compile_fail,E0321
 #![feature(optin_builtin_traits)]