clean up E0468 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0468.md

@@ -1,4 +1,4 @@
-A non-root module attempts to import macros from another crate.
+A non-root module tried to import macros from another crate.
 
 Example of erroneous code:
 
@@ -17,7 +17,7 @@ Either move the macro import to crate root or do without the foreign macros.
 This will work:
 
 ```
-#[macro_use(debug_assert)]
+#[macro_use(debug_assert)] // ok!
 extern crate core;
 
 mod foo {