gui: Fix usage statistics URL in report usage preview (fixes #3397)

This applies the fix from 9d75652 to the usage report preview.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3402
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f