docs: php-fpm grant access (#6227)

php-fpm runs as the apache user, add it to the librenms group too.
Changing the user of php-fpm breaks other things, so this is the better fix
Primarily broken when using rrdcached.

doc/Installation/Installation-CentOS-7-Nginx.md

@@ -74,6 +74,7 @@ systemctl restart php-fpm
 ```bash
 useradd librenms -d /opt/librenms -M -r
 usermod -a -G librenms nginx
+usermod -a -G librenms apache
 ```
 
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