git-config doc: unconfuse an example

One fictitious command "proxy-command" is enclosed inside a double
quote pair, while another fictitious command "default-proxy" is not
in the example, but the quoting does not change anything in the pair
of examples.  Remove the quotes to avoid unnecessary confusion.

Noticed by Michael Haggerty.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-config.txt

@@ -251,7 +251,7 @@ Given a .git/config like this:
 
 	; Proxy settings
 	[core]
-		gitproxy="proxy-command" for kernel.org
+		gitproxy=proxy-command for kernel.org
 		gitproxy=default-proxy ; for all the rest
 
 you can set the filemode to true with