minor symfony#9566 Fix "class_exists" PHP function URL (liviubalan)

This PR was merged into the 2.7 branch.

Discussion
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Fix "class_exists" PHP function URL

Change http://php.net/manual/en/function.class-exists().php to http://php.net/manual/en/function.class-exists.php

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Commits
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e1a885d Fix "class_exists" PHP function URL

Author: xabbuh

doctrine/mapping_model_classes.rst

@@ -93,7 +93,7 @@ be adapted for your case::
         }
     }
 
-Note the :phpfunction:`class_exists()` check. This is crucial, as you do not want your
+Note the :phpfunction:`class_exists` check. This is crucial, as you do not want your
 bundle to have a hard dependency on all Doctrine bundles but let the user
 decide which to use.