[#6120] version specific readme requirements link

Author: xabbuh

reference/requirements.rst

@@ -56,4 +56,4 @@ If you want to use Doctrine, you will need to have PDO installed. Additionally,
 you need to have the PDO driver installed for the database server you want
 to use.
 
-.. _`Requirements section of the README`: https://github.com/symfony/symfony/blob/2.3/README.md#requirements
+.. _`Requirements section of the README`: https://github.com/symfony/symfony/blob/2.7/README.md#requirements