minor symfony#6833 Fixed a syntax issue in custom_constraint article (javiereguiluz)

This PR was merged into the 2.7 branch.

Discussion
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Fixed a syntax issue in custom_constraint article

Here you can see how the render is broken: http://symfony.com/doc/current/validation/custom_constraint.html#constraint-validators-with-dependencies

Commits
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d0128b0 Fixed a syntax issue in custom_constraint article

Author: wouterj

validation/custom_constraint.rst

@@ -167,7 +167,7 @@ Constraint Validators with Dependencies
 If your constraint validator has dependencies, such as a database connection,
 it will need to be configured as a service in the Dependency Injection
 Container. This service must include the ``validator.constraint_validator``
-tag so that the validation system knows about it::
+tag so that the validation system knows about it:
 
 .. configuration-block::