docs/git-gc: fix minor rendering issue

An unwanted single quote character in the paragraph documenting the
'gc.aggressiveWindow' config variable prevented the name of that
config variable from being rendered correctly, ever since that piece
of docs was added in 0d7566a5ba (Add --aggressive option to 'git gc',
2007-05-09).

Remove that single quote.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-gc.txt

@@ -123,7 +123,7 @@ The optional configuration variable `gc.aggressiveWindow` controls how
 much time is spent optimizing the delta compression of the objects in
 the repository when the --aggressive option is specified.  The larger
 the value, the more time is spent optimizing the delta compression.  See
-the documentation for the --window' option in linkgit:git-repack[1] for
+the documentation for the --window option in linkgit:git-repack[1] for
 more details.  This defaults to 250.
 
 Similarly, the optional configuration variable `gc.aggressiveDepth`