Update using-tags.md (spatie#401)

Author: Jorge Murta

docs/basic-usage/using-tags.md

@@ -43,7 +43,7 @@ $yourModel->detachTag('tag 1');
 $yourModel->detachTags(['tag 2', 'tag 3']);
 
 //using an instance of \Spatie\Tags\Tag
-$yourModel->detach(\Spatie\Tags\Tag::Find('tag4'));
+$yourModel->detach(\Spatie\Tags\Tag::find('tag4'));
 ```
 
 ## Syncing tags