Fix lint errors.

client/components/users/userAvatar.js

@@ -17,7 +17,7 @@ Template.userAvatar.helpers({
   fixAvatarUrl(avatarUrl) {
     // Remove suburl from beginning of avatar file path,
     // so that avatar images don't get broken when root-url changes to different sub-url.
-    avatarUrl = '/' + avatarUrl.substring(avatarUrl.indexOf('/cfs/files/avatars/')+1);
+    avatarUrl = `/${  avatarUrl.substring(avatarUrl.indexOf('/cfs/files/avatars/')+1)}`;
     return avatarUrl;
   },