github: Specify mdbook's target path
github: Specify mdbook's target path

As crrev.com/c/3423291 added `linkcheck` target to book.toml, the
directory structure was changed and we need to specify the path to
publish the documentation.

BUG=none
TEST=check on personal github account

Change-Id: Ic30e26d07d98eb21b0132cc4e792b6a054f98427
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3428606
Commit-Queue: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Auto-Submit: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Commit-Queue: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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