minor doc typo on main README.md

[jfass]

the "Source" instructions for cloning the repo are missing the 'bin' directory when you run './barrnap --help' (if you follow the instructions explicitly you'll be in the directory above the bin directory, so that command won't run ... unless someone's added the barrnap bin dir to their path already)

[tseemann]

Thanks Joe - fixed in acf3198