Fix incorrect admin API docs path

[Chandu0501]

Summary

• remove the trailing slash from the /__admin/docs path in the admin API reference
• keep the example URL aligned with the actual WireMock endpoint behavior

Fixes wiremock/wiremock#3332

Verification

• pnpm install
• pnpm approve-builds --all
• pnpm build (Astro rendered dist/docs/standalone/admin-api-reference/index.html with the corrected /__admin/docs URL, but the repo's existing Windows copy-static-homepage hook still fails with an invalid C:\C:\... destination)