Adapt error code doc.

Author: cjgillot

Diff for: compiler/rustc_error_codes/src/error_codes/E0577.md

@@ -3,7 +3,7 @@ Something other than a module was found in visibility scope.
 Erroneous code example:
 
 ```compile_fail,E0577,edition2018
-pub struct Sea;
+pub enum Sea {}
 
 pub (in crate::Sea) struct Shark; // error!