patchbomb: fix inexact description of the messages threading

Each message refers only to the first of the series.

hgext/patchbomb.py

@@ -20,7 +20,7 b' message contains two or three body parts'
 
   The patch itself, as generated by "hg export".
 
-Each message refers to all of its predecessors using the In-Reply-To
+Each message refers to the first in the series using the In-Reply-To
 and References headers, so they will show up as a sequence in threaded
 mail and news readers, and in mail archives.