enable feature gate in E0795.md

Author: GKFX

Diff for: compiler/rustc_error_codes/src/error_codes/E0795.md

@@ -3,7 +3,7 @@ Invalid argument for the `offset_of!` macro.
 Erroneous code example:
 
 ```compile_fail,E0795
-#![feature(offset_of)]
+#![feature(offset_of, offset_of_enum)]
 
 let x = std::mem::offset_of!(Option<u8>, Some);
 ```
@@ -16,7 +16,7 @@ The offset of the contained `u8` in the `Option<u8>` can be found by specifying
 the field name `0`:
 
 ```
-#![feature(offset_of)]
+#![feature(offset_of, offset_of_enum)]
 
 let x: usize = std::mem::offset_of!(Option<u8>, Some.0);
 ```