Fix typo in nss_keylog_export

[lhuang04]

Summary: CLIENT RANDOM -> CLIENT_RANDOM. This appears a typo as it is CLIENT_RANDOM in upstream and in the document.

Test Plan: tests/ssl-opt.sh

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READY/IN DEVELOPMENT/HOLD

Requires Backporting

When there is a bug fix, it should be backported to all maintained and supported branches. Changes do not have to be backported if:

• This PR is a new feature\enhancement
• This PR contains changes in the API. If this is true, and there is a need for the fix to be backported, the fix should be handled differently in the legacy branch

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If there is any API change, what's the incentive and logic for it.

YES | NO

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• [ ] Tests
• [ ] Documentation
• [ ] Changelog updated
• [ ] Backported

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[hannestschofenig]

The change suggested refers to the exporter used with TLS 1.2. However, the label used "CLIENT RANDOM" is not one of the registered labels, see https://datatracker.ietf.org/doc/html/rfc5705 and https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#exporter-labels.

Since even the two key derivation functions are different, I wonder whether a separate code path for 1.2 is required and whether this entry should be removed.