wording fixes.

kame/kame/rtadvd/rtadvd.conf

@@ -16,9 +16,9 @@ ether:\
 	:mtu#1500:tc=default:
 #
 # per-interface definitions.
-#   IPv6 prefixes are mainly configured in this part. However, rtadvd
+#   Mainly IPv6 prefixes are configured in this part. However, rtadvd
 #   automatically learns appropriate prefixes from the kernel's routing
-#   table and advertises the prefix, so you don't have to configure
+#   table and advertises the prefixes, so you don't have to configure
 #   this part, either.
 #   If you don't want the automatic advertisement, invoke rtadvd with
 #   the -s option and configure this part by hand.