Replaced simple comment with doc comment

LeSeulArtichaut · mark-i-m · commit 0e32b4eeefe0 · 2020-01-29T12:19:31.000-06:00
diff --git a/src/implementing_new_features.md b/src/implementing_new_features.md
@@ -129,7 +129,7 @@ a new unstable feature:
    in the active `declare_features` block:
 
    ```rust,ignore
-   // description of feature
+   /// description of feature
    (active, $feature_name, "$current_nightly_version", Some($tracking_issue_number), $edition)
    ```
 
