cros_async: fix unknown function warning in docs
cros_async: fix unknown function warning in docs

Specify the type so cargo doc can find the right async_from function.
Fixes a warning when running tools/cargo-doc.

BUG=None
TEST=tools/cargo-doc

Change-Id: Ida4431cd663d4b5959a58a707c9c89d7569df986
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3764043
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Tested-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm