docs: Fix documentation about using environment variables to obfuscate sensitive values

[lmbaschiera]

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Give us some context

• What problems does this PR solve?
  Adds missing whitespaces between the curly brackets and the environment variable name. Without them, these are interpreted literally.
• Add any context that will help us review your changes such as testing notes,
  In the tests that fixed an issue with variable passthrough, they do have whitespaces https://github.com/newrelic/nri-kubernetes/blob/main/charts/newrelic-infrastructure/tests/configmap_integrations_test.yaml#L231
  We spent quite some time trying to figure out why the variables were not working, only to find out it was due to the whitespaces 😅

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[rhetoric101]

@lmbaschiera I spoke with one of our devlopers and they said they have tests that confirm this syntax should work with or without spaces inside the curly braces. Have you spoken with our support team about this?

[lmbaschiera]

Hi @rhetoric101 - I have not reached out to the support, after some trial and error we figured it was because of the spaces when it worked after adding them. I don't think it was something else.