Set headers in init() so people following the guides don't get eslint errors

[sdhull]

As suggested on the #-team-learning slack channel, this example has been updated so it won't produce eslint errors for people who copy it directly out of the guide.

I added a note in parentheses explaining why, which I was kind of on the fence about including. Happy to update this as the wise learning team sees fit, lmk.

[jenweber]

@sdhull Can you change this to us this.set() instead of just this.headers? After that, it looks good to go.

I err on the side of overexplaining for new people so I think the side note is good to include.

[jenweber]

P.S. thanks for working on this!

[sdhull]

Always looking for ways I can help out 👍 😄

[sdhull]

✅ done

[jenweber]

Super, thank you! So fast 😁 Congrats on your first Guides PR. Merged.