gitweb.conf.txt: switch pluses to backticks to help Asciidoctor

gitweb.conf.txt: switch pluses to backticks to help Asciidoctor

This paragraph uses a lot of +pluses+ to render text as monospace. That
works fine with AsciiDoc (8.6.10), and almost fine with Asciidoctor
(1.5.5), which renders the third of these literally ("+$projname+"). The
reason seems to be that Asciidoctor trips on the lone plus a bit
earlier, even though it is escaped.

Switch +$projname+ to `$projname`, and change the next, similar instance
too (+$projname/+), because otherwise, we'd trip on /that one/ instead.
If we would stop there, we would now start falling over on the escaped
plus ('\+') mentioned earlier, rendering /it/ literally. So change that
too...

In other words, unescape the lone '+' and change all the pluses that
follow it to backticks.

AsciiDoc renders this paragraph identically before and after this
commit, and Asciidoctor now renders this the same as AsciiDoc.

I did try to switch the whole paragraph to using backticks rather than
pluses. That worked great with Asciidoctor, but confused AsciiDoc...
Let's go with this rather surgical change instead.

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