kconfig: doc: fix a typo in the note about 'imply'

This sentence does not make sense due to a typo. Fix it.

Fixes: def2fbff

 ("kconfig: allow symbols implied by y to become m")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>