Update file permissions.md

What does this MR do and why?

In reading the documentation for this page, the phrase "You can assign users a default role or a custom role" could be misunderstood to mean that a role can be designated as the "default" to be assigned to all users upon creation, when the text means "you can assign a custom role to users, or you can use one of the standard GitLab roles (that are listed below).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.