Change markdown header tab anchor links to buttons

What does this MR do?

Changes markdown header tab anchor links to buttons. (see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21988#note_105636972)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

app/assets/stylesheets/framework/markdown_area.scss

@@ -84,14 +84,13 @@
         }
       }
 
-      &.active{
+      &.active {
         button {
           font-weight: 600;
           color: $black;
           border-bottom: 2px solid $theme-gray-800;
         }
       }
-
     }
   }
 }