net: dns_resolver: Fix request-key cross-reference

Link to "Key Request Service" docs uses file:// scheme instead due to
angled brackets markup. Fix it to proper cross-reference.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://patch.msgid.link/20250924020626.17073-4-bagasdotme@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>