Rename instance attachment to match versions 23 and earlier

[cygnus8595]

In versions 23 and earlier, Apigee instance attachments were named -. Later versions swapped that to -, which will cause downtime when upgrading to the latest module as the attachments would have to be destroyed and re-created.

This PR just swaps the names back to how they were to avoid causing downtime for just a name.

Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

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[juliocc]

@apichick can you take a look?

changes made