Fix minor typo in readme

[valeriecodes]

As it was written, generates the following error on bundle install:

[!] There was an error parsing `Gemfile`: You passed :master as an option for gem 'refinerycms-api', but it is invalid. Valid options are: group, groups, git, path, glob, name, branch, ref, tag, require, submodules, platform, platforms, type, source, install_if, github, gist, bitbucket. Bundler cannot continue.```

[bricesanchez]

Thanks !