Update name threadnetwork.country_code in comments

The properity `ro.boot.threadnetwork.country_code` has been renamed
to `threadnetwork.country_code` in aosp/2923770. This CL updates the
properity name in the comment.

Bug: b/309357909
Test: Run `atest ThreadNetworkUnitTests`.
Change-Id: I630f4d970452b17c3ee35fa8e9792ca907341ee8
1 file changed