Correct gen_init_cpio tool's documentation

The documentation for gen_init_cpio mentions to invoke the tool with
'--help' flag for showing directive file format information. However,
only the short name variant '-h' is implemented. Invoking the tool with
the long name returns invalid option message and sets an error code.

Signed-off-by: Andrzej Bednarski <andrzej.bednarski@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>