minor #11291 be consistent ´with the following code example (OskarStark)

This PR was merged into the 3.4 branch.

Discussion
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be consistent ´with the following code example

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Commits
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443e069 be consistent ´with the following code example

Author: javiereguiluz

form/form_collections.rst

@@ -516,6 +516,7 @@ you will learn about next!).
 
         // src/AppBundle/Entity/Task.php
 
+        // ...
         public function addTag(Tag $tag)
         {
             // for a many-to-many association: