docs: update required permissions for editing member flags

[shiftinv]

Summary

https://discord.com/developers/docs/change-log#modify-guild-member-flags-field-permissions

Checklist

• [ ] If code changes were made, then they have been tested
  • [ ] I have updated the documentation to reflect the changes
  • [ ] I have formatted the code properly by running pdm lint
  • [ ] I have type-checked the code by running pdm pyright
• [ ] This PR fixes an issue
• [ ] This PR adds something new (e.g. new method or parameters)
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [x] This PR is not a code change (e.g. documentation, README, ...)