Documentation: build redo-seen.sh from jch..seen

In a similar spirit as the previous commit, the 'seen' branch gets
rebuilt by reintegrating topics between 'jch' and the (old) tip of
'seen'.

Update the instructions on how to generate Meta/redo-seen.sh for the
first time to reflect this.

Signed-off-by: Taylor Blau <me@ttaylorr.com>

Documentation/howto/maintain-git.txt

@@ -298,7 +298,7 @@ by doing the following:
 
  - Rebuild 'seen'.
 
-     $ Meta/Reintegrate master..seen >Meta/redo-seen.sh
+     $ Meta/Reintegrate jch..seen >Meta/redo-seen.sh
 
    Edit the result by adding new topics that are not still in 'seen'
    in the script.  Then