RFC52: rename `LICENSE` subdirectory to `LICENSES`

The REUSE spec requires a subdirectory named `LICENSES`. [1]

Rename it in the RFC text to stay as close to the standard as possible
and to comply with the Principle of Least Surprise. [2]

See also aurweb!854. [3]

[1]: https://reuse.software/spec-3.3/#license-files
[2]: https://en.wikipedia.org/wiki/Principle_of_least_astonishment
[3]: aurweb!854