histedit: correct the number of added revisions in online help

In the context of help document on which this patch focuses, the
example repository, which is source of cloning, should be already
histedit-ed, and contain only 3 revisions (rev # 0 to 2). So, not 3,
but 4 revisions should be added to the destination repository of
cloning, if it contains 7 revisions (rev # 0 to 6).

This patch also adds modifier "histedit-ed" to "example repository",
to make context clear.

hgext/histedit.py

@@ -108,8 +108,8 b' you can use ``hg histedit --abort`` to a'
 have made and return to the state before you attempted to edit your
 history.
 
-If we clone the example repository above and add three more changes, such that
-we have the following history::
+If we clone the histedit-ed example repository above and add four more
+changes, such that we have the following history::
 
    @  6[tip]   038383181893   2009-04-27 18:04 -0500   stefan
    |    Add theta