Add correction to File systems access permissions topic

[ihor-sviziev]

General issue

Affected topic: /guides/v2.4/config-guide/prod/prod_file-sys-perms.html

Description:

Please look at https://github.com/magento/magento2/issues/13225 - this is documentation issue

Possible solutions:

Additional information:

[keharper]

@ihor-sviziev We need clear instructions about what needs to be changed. Reading through the comments of magento/magento2#13225, it looks like a code bug more than a docs bug.

[ihor-sviziev]

@keharper, when js/css merging is enabled - the folder pub/static/_cache even in production mode should be created and writable.