Update kotlinx-io API Reference task

[fzhinkin]

Update the task to work correctly after Dokka update.

See also: Kotlin/kotlinx-io#410

[zoobestik]

@fzhinkin, @whyoleg, guys, when should it be merged? 🙃

[whyoleg]

As kotlinx-io is configured to deploy documentation only from the tag, I think that this PR could be merged right after the PR into kotlinx-io (Kotlin/kotlinx-io#410). In that case, on the next kotlinx-io release, everything should work fine.

[whyoleg]

Note: this PR should be merged when releasing the next version of kotlinx-io, which contains Kotlin/kotlinx-io#410. KOTLINX_IO_RELEASE_LABEL should be set to the next release version (be it 0.8.0 or 0.7.1)