minor #17010 [HttpCache] add validation model ref link (mohamedGasmii)

This PR was merged into the 4.4 branch.

Discussion
----------

[HttpCache] add validation model ref link

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `6.x` for features of unreleased versions).

-->

Commits
-------

e34c9d0 added validation model ref link

Author: javiereguiluz

http_cache/validation.rst

@@ -9,7 +9,7 @@ data, the expiration model falls short. With the `expiration model`_, the
 application won't be asked to return the updated response until the cache
 finally becomes stale.
 
-The validation model addresses this issue. Under this model, the cache continues
+The `validation model`_ addresses this issue. Under this model, the cache continues
 to store responses. The difference is that, for each request, the cache asks the
 application if the cached response is still valid or if it needs to be regenerated.
 If the cache *is* still valid, your application should return a 304 status code
@@ -235,6 +235,7 @@ headers that must not be present for ``304`` responses (see
 :method:`Symfony\\Component\\HttpFoundation\\Response::setNotModified`).
 
 .. _`expiration model`: https://tools.ietf.org/html/rfc2616#section-13.2
+.. _`validation model`: https://tools.ietf.org/html/rfc2616#section-13.3
 .. _`HTTP ETag`: https://en.wikipedia.org/wiki/HTTP_ETag
 .. _`DeflateAlterETag`: https://httpd.apache.org/docs/trunk/mod/mod_deflate.html#deflatealteretag
 .. _`BrotliAlterETag`: https://httpd.apache.org/docs/2.4/mod/mod_brotli.html#brotlialteretag