Fix typo breaking the collapsing of placeholders

Related issue:
- https://github.com/gorhill/uBlock/issues/2848

src/js/contentscript.js

@@ -1166,7 +1166,7 @@ vAPI.injectScriptlet = function(doc, text) {
         if ( collapseToken === undefined ) {
             collapseToken = vAPI.randomToken();
             vAPI.userStylesheet.add(
-                `[${collapseToken}]\ndisplay:none!important;}`,
+                `[${collapseToken}]\n{display:none!important;}`,
                 true
             );
         }