fix(xo-web/backup-ng): typo in the backup form

[badrAZ]

Fixes #3854

Check list

Check items when done or if not relevant

• PR reference the relevant issue (e.g. Fixes #007)
• if UI changes, a screenshot has been added to the PR
• CHANGELOG:
  • enhancement/bug fix entry added
  • list of packages to release updated (${name} v${new version})
• documentation updated

Process

1. create a PR as soon as possible
2. mark it as WiP: (Work in Progress) if not ready to be merged
3. when you want a review, add a reviewer
4. if necessary, update your PR, and re- add a reviewer

[pdonias]

Do you want to add a Changelog entry?

[badrAZ]

Sure 👍