delta-find: small documentation update

This is not a 1-1 mapping, but a 1-n mapping. Lets make the associated comment clearer.

mercurial/revlogutils/deltas.py

@@ -878,7 +878,7 b' def _rawgroups(revlog, p1, p2, cachedelt'
 
     if sparse and parents:
         if snapshots is None:
-            # map: base-rev: snapshot-rev
+            # map: base-rev: [snapshot-revs]
             snapshots = collections.defaultdict(list)
         # See if we can use an existing snapshot in the parent chains to use as
         # a base for a new intermediate-snapshot