resolve_url_loader codeblock did not contain valid javascript

is 
```'resolve_url_loader' => false```
should be
```'resolve_url_loader': false```

Author: kasn

frontend/encore/bootstrap.rst

@@ -42,7 +42,7 @@ this, you can use the ``resolve_url_loader`` option:
     // webpack.config.js
     Encore
     +     .enableSassLoader({
-    +         'resolve_url_loader' => false
+    +         'resolve_url_loader': false
     +     })
     ;