minor #5750 fix YAML syntax highlighting (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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fix YAML syntax highlighting

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #5684

In YAML, double quoted strings can contain escape sequences. Thus, we would have to escape the backslash if we wanted to use them instead of single quotes (see the Platform.sh build to spot the difference now).

Commits
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818d734 fix YAML syntax highlighting

Author: xabbuh

cookbook/email/dev_environment.rst

@@ -143,11 +143,11 @@ by adding the ``delivery_whitelist`` option:
             delivery_whitelist:
                # all email addresses matching this regex will *not* be
                # redirected to [email protected]
-               - "/@specialdomain\.com$/"
+               - '/@specialdomain\.com$/'
 
                # all emails sent to [email protected] won't
                # be redirected to [email protected] too
-               - "/^admin@mydomain\.com$/"
+               - '/^admin@mydomain\.com$/'
 
     .. code-block:: xml