doc: Rename .system_keyring to .builtin_trusted_keys

Commit d3bfe84129f65e0af2450743ebdab33d161d01c9 changed the name but did
not update the documentation.

Fixes: d3bfe84129f65e0af2450743ebdab33d161d01c9
Signed-off-by: Philipp Hahn <hahn@univention.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
1 file changed