Fix case sensitive example

[milliams]

The field name for discard_regex is case sensitive and the example usage does not match the JSON sample.

In the JSON sample, the field is called principalId but the text and the code snippet refer to it as principalID. In reality, AWS logs use the principalId format so this updates the docs to match this.

If someone were to use the example as it is then the events would not be discarded.

Documentation compilation

• [x] Verified that documentation compiles without warnings.

Changelog

• [ ] Updated CHANGELOG.md.