LoadPin: Fix Kconfig doc about format of file with verity digests

The doc for CONFIG_SECURITY_LOADPIN_VERITY says that the file with verity
digests must contain a comma separated list of digests. That was the case
at some stage of the development, but was changed during the review
process to one digest per line. Update the Kconfig doc accordingly.

Reported-by: Jae Hoon Kim <kimjae@chromium.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Fixes: 3f805f8c ("LoadPin: Enable loading from trusted dm-verity devices")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220829174557.1.I5d202d1344212a3800d9828f936df6511eb2d0d1@changeid