fix: remove misleading description

[snorwin]

What type of PR is this?

/kind documentation
/kind cleanup

What this PR does / why we need it:
The description of HTTPHeaderFilter does not accurately reflect the current CEL validation behavior. This PR corrects the description, as changing the CEL validation itself would be a breaking change.

Which issue(s) this PR fixes:

Fixes #3407

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

Hi @snorwin. Thanks for your PR.

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[youngnick]

/ok-to-test
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: snorwin, youngnick

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[robscott]

Thanks @snorwin!

/lgtm