Tweak words in line with style guide

- Update link/spelling of 'User-centred' in line with Service Manual
- Remove full stops around eg
- Fix typo 'interative' -> 'interactive'
- Capitalise Performance Platform
Change frontend to be one word

This is in line with the README in the frontend toolkit repo.
Merge pull request #15 from alphagov/style-guide-tweaks

Thanks for these!