PHP OPcache replaced dead link

https://www.php.net/manual/en/intro.opcache.php is dead
https://www.php.net/manual/en/book.opcache.php is alive

Signed-off-by: Crizomb <62544756+Crizomb@users.noreply.github.com>

Author: Crizomb

admin_manual/installation/server_tuning.rst

@@ -128,7 +128,7 @@ more processes to run in parallel to handle the requests.
 Enable PHP OPcache
 ------------------
 
-The `OPcache <https://php.net/manual/en/intro.opcache.php>`_ improves the performance of PHP applications by caching precompiled bytecode.
+The `OPcache <https://www.php.net/manual/en/book.opcache.php>`_ improves the performance of PHP applications by caching precompiled bytecode.
 
 Revalidation
 ^^^^^^^^^^^^