Fix defmt feature name.

[Dirbaio]

crates.io doesn't allow . in feature names:

the remote server responded with an error: invalid upload request: invalid value: string "defmt-0.3", expected a valid feature name containing only letters, numbers, '-', '+', or '_' at line 1 column 182

[bors[bot]]

This PR was included in a batch that successfully built, but then failed to merge into master. It will not be retried.

Additional information:

Response status code: 422
{"message":"Required status check \"clippy_check\" is in progress.","documentation_url":"https://docs.github.com/articles/about-protected-branches"}

[therealprof]

bors retry

[Dirbaio]

bors r+

[bors[bot]]

Already running a review

[Dirbaio]

bors cancel

[Dirbaio]

bors r+

[bors[bot]]

Build succeeded:

• ci-linux (1.60.0, x86_64-unknown-linux-gnu)
• ci-linux (stable, x86_64-unknown-linux-gnu)