Fixes a typo in the form collector styles

Author: Romain Pierre

src/Symfony/Bundle/WebProfilerBundle/Resources/views/Collector/form.html.twig

@@ -60,7 +60,7 @@
         }
         #tree-menu .empty {
             border: 0;
-            mmargin: 0;
+            margin: 0;
             padding: 0;
         }
         #tree-details-container {