minor #4215 Fixed code highlighting (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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Fixed code highlighting

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

 > Double quoted strings are the only scalar style capable of expressing arbitrary strings, by using “\” escape sequences.
 > - http://yaml.org/spec/1.1/#id904245

`\w` and `\d` are invalid escape sequences in Yaml, causing the Yaml highlighter to fail.

Commits
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f4b5dc6 Fixed code highlighting

Author: weaverryan

reference/constraints/Regex.rst

@@ -32,7 +32,7 @@ characters at the beginning of your string:
         Acme\BlogBundle\Entity\Author:
             properties:
                 description:
-                    - Regex: "/^\w+/"
+                    - Regex: '/^\w+/'
 
     .. code-block:: php-annotations
 
@@ -98,7 +98,7 @@ message:
             properties:
                 firstName:
                     - Regex:
-                        pattern: "/\d/"
+                        pattern: '/\d/'
                         match:   false
                         message: Your name cannot contain a number