Documentation: spufs: correct a duplicate word typo

Fix a typo of "or" which should be "of".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: Jeremy Kerr <jk@ozlabs.org>
Link: https://lore.kernel.org/r/20220829232908.32437-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>