minor #8831 Update collection.rst (babaorum)

This PR was submitted for the 4.0 branch but it was merged into the 3.3 branch instead (closes #8831).

Discussion
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Update collection.rst

A default value is set and seems to work for the `entry_type` option.
https://github.com/symfony/symfony/blob/3ce9c29ae44692555d15983c7537187f5566e9ce/src/Symfony/Component/Form/Extension/Core/Type/CollectionType.php#L97

Commits
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5084ad1 Update collection.rst

Author: javiereguiluz

reference/forms/types/collection.rst

@@ -316,7 +316,7 @@ type::
 entry_type
 ~~~~~~~~~~
 
-**type**: ``string`` or :class:`Symfony\\Component\\Form\\FormTypeInterface` **required**
+**type**: ``string`` or :class:`Symfony\\Component\\Form\\FormTypeInterface` **default**: Symfony\\Component\\Form\\Extension\\Core\\Type\\TextType
 
 This is the field type for each item in this collection (e.g. ``TextType``,
 ``ChoiceType``, etc). For example, if you have an array of email addresses,