minor symfony#17007 [HttpKernel] [HttpCache] Update a link to Varnish cookie docs (javiereguiluz)

This PR was merged into the 4.4 branch.

Discussion
----------

[HttpKernel] [HttpCache] Update a link to Varnish cookie docs

Fixes symfony#16180.

Commits
-------

0960134 [HttpCache] Update a link to Varnish cookie docs

Author: javiereguiluz

http_cache/varnish.rst

@@ -235,7 +235,7 @@ proxy before it has expired, it adds complexity to your caching setup.
 
 .. _`Varnish`: https://varnish-cache.org/
 .. _`Edge Architecture`: http://www.w3.org/TR/edge-arch
-.. _`clean the cookies header`: https://varnish-cache.org/trac/wiki/VCLExampleRemovingSomeCookies
+.. _`clean the cookies header`: https://varnish-cache.org/docs/7.0/reference/vmod_cookie.html
 .. _`Surrogate-Capability Header`: http://www.w3.org/TR/edge-arch
 .. _`cache invalidation`: https://tools.ietf.org/html/rfc2616#section-13.10
 .. _`FOSHttpCacheBundle`: https://foshttpcachebundle.readthedocs.io/en/latest/features/user-context.html