php: raise max_children

Replace an arbitrarily picked value with a higher arbitrarily picked
value.

Author: supertassu

modules/php/templates/fpm/pool.ini.erb

@@ -7,7 +7,7 @@ listen.owner = www-data
 listen.group = www-data
 
 pm = dynamic
-pm.max_children = 5
+pm.max_children = 20
 pm.start_servers = 2
 pm.min_spare_servers = 1
 pm.max_spare_servers = 3