Adds ← to Facebook Product Sets backlink for consistency.

[krutidugade]

Changes proposed in this Pull Request:

I missed adding a backward arrow (←) to the Facebook Product Sets backlink in PR #2581. This PR adds ← to the backlink. Also, I have added a comment for translators explaining what is %s value.

• Do the changed files pass phpcs checks? Please remove phpcs:ignore comments in changed files and fix any issues, or delete if not practical.

Screenshots:

Detailed test instructions:

1. Install and activate Facebook for WooCommerce plugin
2. Go to Marketing > Facebook Product Sets
3. Edit existing Product set (if you have none then add one and then edit it)
4. Make changes and click Save
5. Verify the backlink label in the success notice says "← Go to Facebook Product Sets"

Changelog entry

Tweak - Adds backward arrow to Facebook Product Sets backlink.