Attempt to copyedit the page on APIs

Fix typos, fix bad links.
Tweaks to features-of-agile

- Remove duplication of one sentence.
- I'm assuming Seams is meant to be Scrum? Not sure.
Tweaks to assisted digital action plan

Links in the right place, things named the right thing...
Remove more capitals

Except for the places that I added them. For consistency.
Rewrite a couple of sentences on assisted digital

Based on comments from @jabley.
Clean up messiness
Merge pull request #235 from alphagov/copyedit-minor-stuff

Minor copyedit/bugfixing of stuff