mm/Kconfig: Grammar s/an/a/

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

mm/Kconfig

@@ -20,7 +20,7 @@ config FLATMEM_MANUAL
 
 	  Some users of more advanced features like NUMA and
 	  memory hotplug may have different options here.
-	  DISCONTIGMEM is an more mature, better tested system,
+	  DISCONTIGMEM is a more mature, better tested system,
 	  but is incompatible with memory hotplug and may suffer
 	  decreased performance over SPARSEMEM.  If unsure between
 	  "Sparse Memory" and "Discontiguous Memory", choose