Update documentation example for preferredChain

[jnoordsij]

The documentation on the preferredChain still mentions the cross-signed version of ISRG Root X1 CA cert from Let's Encrypt, which has expired as of Monday, September 30th, 2024 (see https://letsencrypt.org/2023/07/10/cross-sign-expiration/).

To use a more representative example, I've replaced this with the example from current Let's Encrypt documentation (see https://letsencrypt.org/certificates/#chains).

Note that the wording and example are very much opiniated, so there's a variety of alternative approaches to replacing this example:

• word differently from what I've now picked
• remove the example entirely
• just refer to upstream Let's Encrypt documentation without listing an example, to prevent it from going outdated once more at some point in the future

[cert-manager-prow]

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