docs: fix README.md TestMain example

[fabriziosestito]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Fixes the README.md TestMain example which was using a deprecated API and removing a namespace that did not exist.

Which issue(s) this PR fixes:

NONE

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., Usage docs, etc.:

- [Other doc]: https://github.com/kubernetes-sigs/e2e-framework/blob/main/README.md

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: fabriziosestito
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[linux-foundation-easycla]

• ✅login: fabriziosestito / (2f870a9)

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[k8s-ci-robot]

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