fix: improved scaffolding for custom functions

[davenewza]

Scaffolding for custom functions more consistent with keel generate

Before:

import { DoSomething } from '@teamkeel/sdk';
export default DoSomething(async (ctx, event) => {

})

After:

import { DoSomething } from '@teamkeel/sdk';

// To learn more about what you can do with custom functions, visit https://docs.keel.so/functions
export default DoSomething(async (ctx, event) => {

});