minor #4979 require_once path fixed (mvanmeerbeck)

This PR was merged into the 2.3 branch.

Discussion
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require_once path fixed

added the php extension to the require

Commits
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406313d require_once path fixed

components/class_loader/map_class_loader.rst

@@ -25,7 +25,7 @@ Usage
 Using it is as easy as passing your mapping to its constructor when creating
 an instance of the ``MapClassLoader`` class::
 
-    require_once '/path/to/src/Symfony/Component/ClassLoader/MapClassLoader';
+    require_once '/path/to/src/Symfony/Component/ClassLoader/MapClassLoader.php';
 
     $mapping = array(
         'Foo' => '/path/to/Foo',