Link to fmt license file is broken.

[hvdbrand]

The link to the fmt license file is broken, because the .rst extension is dropped.
This is also because the mentioned license file is now pointing to a rolling release.
I think it would be more appropriate to point to the license for the included release (which is 9.1.0 if I am not mistaken).

[tt4g]

Thank you for reporting.
PR is welcome.