Correct date now docs to reflect current behavior.

[kidrigger]

date now command documentation shows how to correctly output RFC3339 format.
format date command documentation shows how to format to RFC3339 format.

FIXES: nushell/nushell#15168

[github-actions]

Awesome that you are trying to improve the documentation of a nushell command!

You should not modify the files in commands/docs or commands/categories!
We autogenerate these command pages from the help information defined by the command implementation, after each release. Thus:

• If you already updated the command help, you are already done and can close this PR!
• Else, please look for the command in the source tree and make your changes directly there in the impl Command and Signature.
• If you accidentally made a change in commands/ while working on general documentation, please revert changes to commands/

Thanks for helping out!