The help text for CONFIG_PGTABLE_MAPPING has an incorrect URL.  While
we're at it, remove the unnecessary footnote notation.
Signed-off-by: NBen Hutchings <ben@decadent.org.uk>
Acked-by: NMinchan Kim <minchan@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>