bug symfony#7130 [Validator] Fixed incorrect xml constraint example (acrobat)

This PR was merged into the 2.7 branch.

Discussion
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[Validator] Fixed incorrect xml constraint example

I've fixed an incorrect xml example for the type constraint. See image for validation error in phpstorm

![example_bug](https://cloud.githubusercontent.com/assets/1374857/20151627/1b845fc0-a6bb-11e6-94d6-589cf117b8d1.png)

Commits
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786555d Fixed incorrect xml constraint example

Author: wouterj

reference/constraints/Type.rst

@@ -72,7 +72,7 @@ This will check if ``firstName`` is of type ``string`` and that ``age`` is an
             <class name="AppBundle\Entity\Author">
                 <property name="firstName">
                     <constraint name="Type">
-                        <type>string</type>
+                        <option name="type">string</option>
                     </constraint>
                 </property>
                 <property name="age">