Update README.md

[ndouglas]

What does this PR do?

PermissionBoundaryArn -> PermissionsBoundaryArn.

The latter is correct at least as of 3.115.0, unless I'm missing something.

Motivation

Got an error when I specified PermissionBoundaryArn.

Testing Guidelines

didn't test, lol

Additional Notes

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)
• This PR passes the unit tests
• This PR passes the installation tests (ask a Datadog member to run the tests)