commands: fix update's help to be more accurate

Branches can have multiple heads, so it doesn't make sense to speak of "the
head of the current branch". What update really does is try to update to the
*tip* of the current branch.

mercurial/commands.py

@@ -3779,8 +3779,8 def update(ui, repo, node=None, rev=None
     Update the repository's working directory to the specified
     changeset.
 
-    If no changeset is specified, attempt to update to the head of the
-    current branch. If this head is a descendant of the working
+    If no changeset is specified, attempt to update to the tip of the
+    current branch. If this changeset is a descendant of the working
     directory's parent, update to it, otherwise abort.
 
     The following rules apply when the working directory contains