Typo fix

Author: jesusprubio

compiler/rustc_error_codes/src/error_codes/E0546.md

@@ -19,10 +19,10 @@ To fix the issue you need to provide the `feature` field.
 #![feature(staged_api)]
 #![stable(since = "1.0.0", feature = "test")]
 
-#[unstable(feature = "unstable_fn", issue = "none")] // ok!!
+#[unstable(feature = "unstable_fn", issue = "none")] // ok!
 fn unstable_fn() {}
 
-#[stable(feature = "stable_fn", since = "1.0.0")] // ok!!
+#[stable(feature = "stable_fn", since = "1.0.0")] // ok!
 fn stable_fn() {}
 ```