gitworkflows.txt: fix broken subsection underline

AsciiDoctor renders the "~~~~~~~~~" literally. That's not our intention:
it is supposed to indicate a level 2 subsection. In 828197de8f ("docs:
adjust for the recent rename of `pu` to `seen`", 2020-06-25), the length
of this section header grew by two characters but we didn't adjust the
number of ~ characters accordingly. AsciiDoc handles this discrepancy ok
and still picks this up as a subsection title, but Asciidoctor is not as
forgiving.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitworkflows.txt

@@ -292,7 +292,7 @@ described in the previous section.
 
 
 Branch management for next and seen after a feature release
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 After a feature release, the integration branch 'next' may optionally be
 rewound and rebuilt from the tip of 'master' using the surviving