Fix namespace path in pending members page

What does this MR do and why?

Fix namespace path in pending members page

The pending members was not showing the pending members due to the endpoint expecting a "namespace path" but we passed "namespace name" instead.

more details

It wasn't caught before because most of our local groups are single words (e.g. Twitter), but when the group is more than one word there will be differences and the page won't list the pending members properly.

for even more details

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

Before	After

How to set up and validate locally

1. Ensure you're running GDK with SAAS enabled (e.g. export GITLAB_SIMULATE_SAAS=1 gdk restart)
2. With a group with an Ultimate plan
3. Navigate to Settings > Permissions, LFS, 2FA > User Cap
4. Set the user cap limit to 1
5. Navigate to the group members
6. Add one or more members (until you are above the user cap limit set in previous step)
7. Navigate to Usage Quota from the left sidebar under Settings
8. You should see an alert that you have pending members
9. Clicking the button in the alert should navigate to a pending members page

Related to #464444