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>

Documentation/gitweb.conf.txt

@@ -786,9 +786,9 @@ forks::
 	subdirectories of project root (basename) to be forks of existing
 	projects.  For each project +$projname.git+, projects in the
 	+$projname/+ directory and its subdirectories will not be
-	shown in the main projects list.  Instead, a \'\+' mark is shown
-	next to +$projname+, which links to a "forks" view that lists all
-	the forks (all projects in +$projname/+ subdirectory).  Additionally
+	shown in the main projects list.  Instead, a \'+' mark is shown
+	next to `$projname`, which links to a "forks" view that lists all
+	the forks (all projects in `$projname/` subdirectory).  Additionally
 	a "forks" view for a project is linked from project summary page.
 +
 If the project list is taken from a file (+$projects_list+ points to a