include/bitmap.h: add missing parameter in docs

bitmap_find_next_zero_area_off() has an additional parameter which was
not specified in the list of functions. Add it.

Fixes: 5e19b013 ("lib: bitmap: add alignment offset for bitmap_find_next_zero_area()")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>