wic: partition.py: fix help string

wic: partition.py: fix help string

The help suggests invoking 'wic help source-plugins' for information on how to
add a new plugin, this results in:

	usage: wic help [-h] {plugins,overview,kickstart,create,ls,cp,rm,write,list} ...
	wic help: error: argument help_topic: invalid choice: 'source-plugins' (choose from 'plugins', 'overview', 'kickstart', 'create', 'ls', 'cp', 'rm', 'write', 'list')

The proper command is 'wic help plugins'.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>