docs: Ecto 3 example update to better guide setups with multiple repos

[rraub]

The first argument to :telemry.attach/4 needs to be unique otherwise it will return an error tuple saying it is already attached. I ran into this issue in an application that has multiple Ecto Repos and I didn't update the first argument to be unique. Which took me a while to track down why I was getting data for one of my repos but not the other.

I figure if the example includes something that should be unique per repo name that may help others with multiple repos.

Alternatively we could include a match so it would fail the match on assignment :ok = :telemry.attach(... I figured the intention isn't as clear though.

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[zachdaniel]

@rraub sorry it took ages to get back to you, I think I'm not getting notifications for this repo. I think the pattern match :ok = is actually a good idea in addition to this update.

[rraub]

@zachdaniel No worries, I agree and have added that pattern match.

[zachdaniel]

🚀 Thank you for your contribution! 🚀