gc docs: fix formatting for "gc.writeCommitGraph"

Change the AsciiDoc formatting so that an example of "gc --auto" isn't
rendered as "git-gc(1) --auto", but as "git gc --auto". This is
consistent with the rest of the links and command examples in this
documentation.

The formatting I'm changing was initially introduced in
d5d5d7b641 ("gc: automatically write commit-graph files", 2018-06-27).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/gc.txt

@@ -59,8 +59,8 @@ will also be excluded (this is the equivalent of running `git gc` with
 
 gc.writeCommitGraph::
 	If true, then gc will rewrite the commit-graph file when
-	linkgit:git-gc[1] is run. When using linkgit:git-gc[1]
-	'--auto' the commit-graph will be updated if housekeeping is
+	linkgit:git-gc[1] is run. When using `git gc --auto`
+	the commit-graph will be updated if housekeeping is
 	required. Default is false. See linkgit:git-commit-graph[1]
 	for details.