devlink: remove trigger command from devlink-region.rst

The devlink trigger command does not exist. While rewriting the
documentation for devlink into the reStructuredText format,
documentation for the trigger command was accidentally merged in. This
occurred because the author was also working on a potential extension to
devlink regions which included this trigger command, and accidentally
squashed the documentation incorrectly.

Further review eventually settled on using the previously unused "new"
command instead of creating a new trigger command.

Fix this by removing mention of the trigger command from the
documentation.

Fixes: 0b0f945f ("devlink: add a file documenting devlink regions", 2020-01-10)
Noticed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>