Tweaks suggested to original pull request

See comments on #131 

* readability
* hyphenate

This does not yet address @fatbusinessman's [first comment](https://github.com/alphagov/government-service-design-manual/pull/131#discussion-diff-4278219). I'm not sure what the intent of that was, and would probably go back to @norm for clarification. Perhaps something like:

```
There are a number of common misunderstandings when designing progressively enhanced web sites. It is not about:

* x
* y
* z
```