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>

Documentation/early-userspace/README

@@ -52,7 +52,7 @@ user root (0).  INITRAMFS_ROOT_GID can be set to a group ID that needs
 to be mapped to group root (0).
 
 A source file must be directives in the format required by the
-usr/gen_init_cpio utility (run 'usr/gen_init_cpio --help' to get the
+usr/gen_init_cpio utility (run 'usr/gen_init_cpio -h' to get the
 file format).  The directives in the file will be passed directly to
 usr/gen_init_cpio.