Update index.rst

The current link for "Annotated Routes Activation" may be correct according to RST syntax, but in practice it just does not work both at github and at the current live (html) docs:

https://github.com/sensiolabs/SensioFrameworkExtraBundle/blob/master/Resources/doc/index.rst

http://symfony.com/doc/current/bundles/SensioFrameworkExtraBundle/index.html