Improve E0567 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0567.md

@@ -6,8 +6,7 @@ Erroneous code example:
 #![feature(optin_builtin_traits)]
 
 auto trait Generic<T> {} // error!
-
-fn main() {}
+# fn main() {}
 ```
 
 Since an auto trait is implemented on all existing types, the
@@ -20,6 +19,5 @@ To fix this issue, just remove the generics:
 #![feature(optin_builtin_traits)]
 
 auto trait Generic {} // ok!
-
-fn main() {}
+# fn main() {}
 ```