Corrected typo in output options in Readme.md

[bastiRe]

The output options output-image and output-style had an underscore in the Readme.

[jakesgordon]

Ah. There is actually an inconsistency - the option to the underlying ruby method uses an underscore, but the option to the command line sf utility uses a dash. I'll change the sf command line script to accept the underscore version of those options to be consistent with the ruby method and then the docs will be correct.

Thanks