docs(usage): Use express server

[SgtPooki]

The original instructions were outdated and incorrect and started up two servers: one express server inside the mock-ipfs-pinning-service's setup call, and one via http.

This resulted in any requests going to the http server, which has no logic/code behind it and would timeout on requests.

[lidel]

Thanks!

Small nit: this is a lot of code to put in README 🙈 – any way we can condense this a bit?

If not, perhaps "CLI Usage" above "Code Usage" as it is what most people will use on CI anyway.

[SgtPooki]

@lidel how does it look now? see a9ef703

[lidel]

Thanks!