bug #4362 Missing apostrophe in source example. (astery)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4362).

Discussion
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Missing apostrophe in source example.

Commits
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6c10845 Missing apostrophe in source example.

Author: wouterj

Diff for: cookbook/doctrine/dbal.rst

@@ -166,7 +166,7 @@ mapping type:
         // app/config/config.php
         $container->loadFromExtension('doctrine', array(
             'dbal' => array(
-               mapping_types' => array(
+               'mapping_types' => array(
                   'enum'  => 'string',
                ),
             ),