Format fixes

compiler/rustc_error_codes/src/error_codes/E0542.md

@@ -1,6 +1,5 @@
 The `since` value is missing in a stability attribute.
 
-
 Erroneous code example:
 
 ```compile_fail,E0542
@@ -20,7 +19,7 @@ fn _stable_const_fn() {}
 fn _deprecated_fn() {}
 ```
 
-To fix the issue you need to provide the since field of the feature.
+To fix the issue you need to provide the `since` field.
 
 ```
 #![feature(staged_api)]