Documentation: fix typo in root= kernel parameter description

Fixes a typo in the root= parameter description, changing
"this a a" to "this is a".

Fixes: c0c1a7dc ("init: move the nfs/cifs/ram special cases out of name_to_dev_t")
Signed-off-by: Petr Vaněk <arkamar@atlas.cz>
Link: https://lore.kernel.org/20250512110827.32530-1-arkamar@atlas.cz


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>