mm/migrate: correct kernel-doc notation

Use the expected "Return:" format to prevent a kernel-doc warning.

mm/migrate.c:1157: warning: Excess function parameter 'returns' description in 'next_demotion_node'

Link: https://lkml.kernel.org/r/20210808203151.10632-1-rdunlap@infradead.org


Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>