fix(docs): update custom fetch example

[jacobwgillespie]

What kind of change does this PR introduce?

This updates the documentation example of providing a custom fetch implementation (introduced in #297). While the previous example was conceptually correct, in most environments it's not possible to assign a native built-in function like fetch to an object, as calling it later will attempt to call the native function in the context of the object and throw an illegal invocation error.

Creating a new fetch function by binding to the global scope, for instance fetch.bind(global) in Node or fetch.bind(self) in Cloudflare Workers, fixes this issue, but creating a new arrow function is the most univerally functional example since it doesn't require you to know if global is defined or not, so this PR uses that as an example:

{
  fetch: (...args) => fetch(...args),
}

Additional context

This is followup to #297.