minor #4132 Use proper way to reference a doc page for legacy sessions (Baptouuuu)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4132).

Discussion
----------

Use proper way to reference a doc page for legacy sessions

| Q                  | A
| ---------------- | ---
| Doc fix?        | yes
| New docs?   | no
| Applies to     | 2.5+
| Fixed tickets |

Commits
-------

ec04c2a use relative path to page

Author: weaverryan

components/http_foundation/sessions.rst

@@ -16,7 +16,7 @@ implementation of :class:`Symfony\\Component\\HttpFoundation\\Session\\SessionIn
 
     Make sure your PHP session isn't already started before using the Session
     class. If you have a legacy session system that starts your session, see
-    http://symfony.com/doc/current/components/http_foundation/session_php_bridge.html
+    :doc:`Legacy Sessions </components/http_foundation/session_php_bridge>`.
 
 Quick example::