minor #6364 [reference] [constraints] added missing colon character for 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
----------

[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
-------

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

Author: xabbuh

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: