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/version.c b/src/version.c
index 13dff53..5d935fe 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    640,
+/**/
     639,
 /**/
     638,