minor #16698 [Doctrine] Fixing link syntax (ThomasLandauer)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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[Doctrine] Fixing link syntax

...I hope ;-)
It's rendered broken on https://symfony.com/doc/4.4/doctrine/custom_dql_functions.html

Commits
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fbb9b93 [Doctrine] Fixing link syntax

Author: javiereguiluz

doctrine/custom_dql_functions.rst

@@ -5,7 +5,7 @@ How to Register custom DQL Functions
 ====================================
 
 Doctrine allows you to specify custom DQL functions. For more information
-on this topic, read Doctrine's cookbook article "`DQL User Defined Functions`_".
+on this topic, read Doctrine's cookbook article `DQL User Defined Functions`_.
 
 In Symfony, you can register your custom DQL functions as follows: