[HttpCache] add validation model ref link

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