Improved 0.20 to 0.21 migration guide.

[taylorzane]

While migrating my app I noticed a very minor issue in the diff example. I imagine someone else may run into this down the road.

I wasn't sure whether to do A or B. Let me know if you'd like me to switch the diff.

A (as it is):

- sirv('assets', { dev }),
+ sirv('static', { dev }),
- sapper({ manifest })
+ sapper.middleware()

B:

- sirv('assets', { dev }),
- sapper({ manifest })
+ sirv('static', { dev }),
+ sapper.middleware()

[Rich-Harris]

Thank you! Either diff is fine I reckon