Doc: Contrib Guide 3.3.5 Patches

[Greg]

I believe that the first code block on 
http://lilypond.org/doc/v2.19/Documentation/contributor/patches is slightly 
incorrect.

On the LilyDev I just downloaded, you need to run `git format-patch 
origin/master`. In addition to the code block being changed, the paragraph that 
follows should therefore read "The origin/master argument" rather than "The 
origin argument".

I didn't want to make the changes myself in case this is different for 
different versions of Git.

Thanks,
Greg.