Fix gate check when updating user permissions on a team

[drewrawitz]

I think I found a bug where managing a team member's role is being guarded by the canAddTeamMembers permissions instead of canUpdateTeamMembers.

This isn't super obvious right off the bat because they share the exact same logic, which is:

return $user->ownsTeam($team);

I spotted this in my own application when I changed the logic a bit so that both Administrators and Owners can invite new team members, but only Owners should be able to modify user permissions.