Wrong multiline default for description from snapcraft init

[Adam Collard]

The default description one gets from `snapcraft init` uses

```
description: |
  This is my-snap's description. You have a paragraph or two to tell the
  most important story about your snap. Keep it under 100 words though,
  we live in tweetspace and your description wants to look good in the snap
  store.
```

Which, as per http://yaml-multiline.info/, will interpret the new lines as hard new lines, and not just wrapped words.

Using > instead of | will treat them as spaces.

[Adam Collard (adam-collard)]

https://github.com/snapcore/snapcraft/pull/2393