Fix incorrect commas in URLs

In Texinfo, a comma inside an @url separates parameters to the @url
function rather than being included as part of a parameter. Use @comma{}
instead to resolve this.

Change-Id: I8b38939462cf4452e5bc2582ee484220aaf83ae0
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/5028
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>