Indent protocol.sgml

protocol.sgml has some unusual indentation that keeps getting added on
to with new additions in that file. I have gone through and reindented
it to be more in line with the style elsewhere, so future editing
hopefully won't be that painful on the eye. ;-)

The diff can be seen here:
https://github.com/petere/postgresql/compare/master...petere:doc-protocol-indent.patch

I'm not aware of any pending patches in this area. I propose to apply
this around the time we do pgindent.