mm/Kconfig: fix URL for zsmalloc benchmark

The help text for CONFIG_PGTABLE_MAPPING has an incorrect URL.  While
we're at it, remove the unnecessary footnote notation.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>