CLEANUP: Fix typo in ARGT_MSK6 comment

The incorrect comment was introduced in commit:
2ac5718dbd4ec722ece228e9f613d2be74eee9da

v1.5-dev9 is the first tag containing this comment, the fix
should be backported to haproxy 1.5 and newer.
diff --git a/include/types/arg.h b/include/types/arg.h
index 651164b..c3b09de 100644
--- a/include/types/arg.h
+++ b/include/types/arg.h
@@ -49,7 +49,7 @@
 	ARGT_IPV4,     /* an IPv4 address */
 	ARGT_MSK4,     /* an IPv4 address mask (integer or dotted), stored as ARGT_IPV4 */
 	ARGT_IPV6,     /* an IPv6 address */
-	ARGT_MSK6,     /* an IPv6 address mask (integer or dotted), stored as ARGT_IPV4 */
+	ARGT_MSK6,     /* an IPv6 address mask (integer or dotted), stored as ARGT_IPV6 */
 	ARGT_TIME,     /* a delay in ms by default, stored as ARGT_UINT */
 	ARGT_SIZE,     /* a size in bytes by default, stored as ARGT_UINT */
 	ARGT_FE,       /* a pointer to a frontend only */