idr: fix param name in idr_alloc_cyclic() doc

The relevant parameter is 'start' and not 'nextid'

Fixes: 460488c5

 ("idr: Remove idr_alloc_ext")
Signed-off-by: Ariel Marcovitch <arielmarcovitch@gmail.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>