docs(readme): use simple english

[Fdawgs]

As much as this pains me, Fastify docs use American English, so we should standardise around that.
This PR replaces British/Traditional English spellings with American/Simple English.

Checklist

• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[jsumners]

I think they are interchangeable. I don't particularly see a reason to be pedantic about the flavor.