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>

mm/Kconfig

@@ -575,5 +575,5 @@ config PGTABLE_MAPPING
 	  then you should select this. This causes zsmalloc to use page table
 	  mapping rather than copying for object mapping.
 
-	  You can check speed with zsmalloc benchmark[1].
-	  [1] https://github.com/spartacus06/zsmalloc
+	  You can check speed with zsmalloc benchmark:
+	  https://github.com/spartacus06/zsmapbench