Fix MR options button visibility at desktop screen size

This commit fixes a previous one that accidentally removed
`d-md-none` from the MR Options dropdown button

app/views/projects/merge_requests/_mr_title.html.haml

@@ -25,7 +25,7 @@
 
   .detail-page-header-actions.js-issuable-actions
     .clearfix.issue-btn-group.dropdown
-      %button.gl-button.btn.btn-default.float-left{ type: "button", data: { toggle: "dropdown" } }
+      %button.gl-button.btn.btn-default.float-left.gl-display-md-none{ type: "button", data: { toggle: "dropdown" } }
         Options
         = sprite_icon('chevron-down', css_class: 'gl-text-gray-500')
       .dropdown-menu.dropdown-menu-right