Fix syntax error in gr-app.js

I got this

Expected ':'

from the console log.

The syntax error broke Internet Explorer support again, so re do :
function should fix it.

Follows up
4d22c7e835 (diff-293fdf98b9f52b1c261c3d06cce2ccc0)

Change-Id: If5a52dc0e2c8808a3b9ceb499a20bf6f62617d9a

polygerrit-ui/app/elements/gr-app.js

@@ -198,7 +198,7 @@
       }
     },
 
-    _showKeyboardShortcuts(e) {
+    _showKeyboardShortcuts: function(e) {
       if (this.shouldSuppressKeyboardShortcut(e)) { return; }
       this.$.keyboardShortcuts.open();
     },