user-manual: be consistent in illustrations to 'git rebase'

Since we use a-b-c for mywork commits in one place, I think it would be
logical to also use a-b-c too in other illustration on this topic.
Signed-off-by: NKirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -2439,9 +2439,9 @@ You have performed no merges into mywork, so it is just a simple linear
 sequence of patches on top of "origin":
 
 ................................................
- o--o--o <-- origin
+ o--o--O <-- origin
         \
-         o--o--o <-- mywork
+	 a--b--c <-- mywork
 ................................................
 
 Some more interesting work has been done in the upstream project, and