format-patch doc: Thunderbird wraps lines unless mailnews.wraplength=0

The Thunderbird section of the 'MUA-specific hints' contains three different approaches to setting up the mail client to leave patch emails unmolested. The second approach (configuration) has a step missing when configuring the composition window not to wrap. In particular, the "mailnews.wraplength" configuration variable needs to be set to zero. Update the documentation to add the missing setting. Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>