Fixed small typo in docs

I found a small typo in the docs while trying to get up to speed on this project, and fixed the sentence to read better. I reviewed the 'guidelines for contributing' and therefore made a separate branch on my fork, before submitting this pull request.  I didn't run the tests, as this is such a small edit. I also reviewed the GitHub article 'Using pull requests'.  This is my first ever pull request, so please let me know if there's any mistakes I've made in putting this together.