Clean up E0458 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0458.md

@@ -1,4 +1,6 @@
-An unknown "kind" was specified for a link attribute. Erroneous code example:
+An unknown "kind" was specified for a link attribute.
+
+Erroneous code example:
 
 ```compile_fail,E0458
 #[link(kind = "wonderful_unicorn")] extern {}