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>