Update curl installer message for anonymized usage collection

[hpuac]

While installing rover I noticed that the message printed for disabling the anonymized usage collection was still suggesting to set APOLLO_TELEMETRY_DISABLED=1.

But since the release v0.18.1 the recommendation was changed to use the boolean flag.

This PR updates the message for the curl installer.
For the npm installer this message was already correct/updated.

[apollo-cla]

@hpuac: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[hpuac]

Signed ✅

[svc-apollo-docs]

✅ Docs preview has no changes

The preview was not built because there were no changes.

Build ID: e70250aba6d49737ffa232c7

[hpuac]

Today marks the one-year anniversary of this open PR. I hope you allow me a sarcastic comment:
Thank you Apollo for taking Open Source so serious and supporting individual contributors as much as you do. Even if the change is trivial I like how much you encourage contributors and support them which increases their motivation to do even more changes and contribute to your business. This is a true inspiration for the whole tech industry, thank you!
/sarcasm off
I will not try to keep this PR up to date anymore. Take them over or close the PR, both is fine for me, in the end it's just two lines changed.