Replace `composer serve` with `composer server start`

[mikelittle]

There are a couple of places in the developer documentation which refer to composer serve as a valid command to start local server.

In fact, it only works because composer will run the a non-conflicting command which starts with what you type. So when you type composer serve it can safely assume you mean composer server. Additionally, when you run composer server without any sub-commands it defaults to composer server start anyway.

I'd rather we documented the command correctly rather than rely on a couple of coincidences that allow it to work.

Files:

• https://github.com/humanmade/altis-documentation/blob/master/other-docs/getting-started/README.md two occurrences
• https://github.com/humanmade/altis-documentation/blob/master/other-docs/guides/migrating/from-hm-platform.md - one occurrence

In all cases composer serve should be replaced with composer server start