docs(dev): Access features as functions, not members

This was changed in rust-lang#132027

Author: epage

src/doc/rustc-dev-guide/src/implementing_new_features.md

@@ -167,7 +167,7 @@ a new unstable feature:
 
 1. Prevent usage of the new feature unless the feature gate is set.
    You can check it in most places in the compiler using the
-   expression `tcx.features().$feature_name`
+   expression `tcx.features().$feature_name()`
 
     If the feature gate is not set, you should either maintain
     the pre-feature behavior or raise an error, depending on