fix: Fix "assigned groups" showing empty when trying to assign groups to a role

[rolodato]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

When the API and FE are running on the same domain, the FE tries to hit /my-groups which is not a valid API endpoint. This causes the "Assigned groups" dropdown to show as empty:

This change adds the missing trailing slash, which makes this work:

How did you test this code?

Manually by running the FE locally against the API on the same domain.

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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4756	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4756	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-4756	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4756	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4756	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-4756	Finished ✅	Results ✅

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