Clean up E0554 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0554.md

@@ -1,7 +1,7 @@
 Feature attributes are only allowed on the nightly release channel. Stable or
 beta compilers will not comply.
 
-Example of erroneous code (on a stable compiler):
+Erroneous code example:
 
 ```ignore (depends on release channel)
 #![feature(non_ascii_idents)] // error: `#![feature]` may not be used on the