minor #6364 [reference] [constraints] added missing colon character f…

…or Image constraint documentation in YAML format. (hhamon)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #6364).

Discussion
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[reference] [constraints] added missing colon character for Image constraint documentation in YAML format.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | ~

Commits
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0588a82 [reference] [constraints] added missing colon character for Image constraint documentation in YAML format.

reference/constraints/Image.rst

@@ -89,7 +89,7 @@ that it is between a certain size, add the following:
     .. code-block:: yaml
 
         # src/AppBundle/Resources/config/validation.yml
-        AppBundle\Entity\Author
+        AppBundle\Entity\Author:
             properties:
                 headshot:
                     - Image: