docs(architecture): update wording

[jvandemo]

This PR updates the wording so it aligns with the actual code that is displayed under the wording.

Before:
Here's a HeroService that fetches heroes and returns them in a resolved Promise.

After:
Here's a HeroService that uses a Promise to fetch heroes.

Code:

getHeroes() {
  this.backend.getAll(Hero).then( (heroes: Hero[]) => {
    this.logger.log(`Fetched ${heroes.length} heroes.`);
    this.heroes.push(...heroes); // fill cache
  });
  return this.heroes;
}

The code does not return a promise so I believe the new wording is more accurate.