Documentation/technical/protocol-v2.txt: fix formatting

Documentation/technical/protocol-v2.txt: fix formatting

Asciidoctor versions v1.5.7 or later print the following warning while
building the documentation:

      ASCIIDOC technical/protocol-v2.html
  asciidoctor: WARNING: protocol-v2.txt: line 38: unterminated listing block

This highlights an issue (even with older Asciidoctor versions) where
the 'Initial Client Request' header is not rendered as a header but in
monospace.  I'm not sure what exactly causes this issue and why it's
an issue only with this particular header, but all headers in
'protocol-v2.txt' are written like this:

   Initial Client Request
  ------------------------

i.e. the header itself is indented by a space, and the "underline" is
two characters longer than the header.

Dropping that indentation and making the length of the underline match
the length of the header apparently fixes this issue.

While at it, adjust all other headers 'protocol-v2.txt' as well, to
match the style we use everywhere else.

The page rendered with AsciiDoc doesn't have this formatting issue.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>