hdajackretask: Update kernel doc URL in README

[GitHub pull_request - opened]

alsa-project/alsa-tools pull request #2 was opened from ferdnyc:

As of THIS writing, the kernel documentation tree has been rearranged, and the "Hint strings" section now resides at the updated URL

Request URL   : https://github.com/alsa-project/alsa-tools/pull/2
Patch URL     : https://github.com/alsa-project/alsa-tools/pull/2.patch
Repository URL: https://github.com/alsa-project/alsa-tools