defmt_itm: Fix typo
## Support
-`defmt-rtt` is part of the [Knurling] project, [Ferrous Systems]' effort at
+`defmt-itm` is part of the [Knurling] project, [Ferrous Systems]' effort at
improving tooling used to develop for embedded systems.
If you think that our work is useful, consider sponsoring it via [GitHub
