patch 8.0.0640: mismatch between help and actual message

Problem:    Mismatch between help and actual message for ":syn conceal".
Solution:   Change the message to match the help. (Ken Takata)
diff --git a/src/syntax.c b/src/syntax.c
index 10805f4..225853e 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -3436,9 +3436,9 @@
     if (*arg == NUL)
     {
 	if (curwin->w_s->b_syn_conceal)
-	    MSG(_("syn conceal on"));
+	    MSG(_("syntax conceal on"));
 	else
-	    MSG(_("syn conceal off"));
+	    MSG(_("syntax conceal off"));
     }
     else if (STRNICMP(arg, "on", 2) == 0 && next - arg == 2)
 	curwin->w_s->b_syn_conceal = TRUE;