Fix exemplar docs.

[JokerQueue]

The structure of the config "storage. exemplars" is not a list.

storage:
  exemplars:
    max_exemplars: 100000

So I think the doc should be

[ exemplars: <exemplars> ]

but not

[ - <exemplars> ... ]

[Nexucis]

Hello, thanks for your contribution.

DCO failed because for legal reason we are asking to contributor to signoff every commits. Can you follow the procedure indicated by DCO https://github.com/prometheus/prometheus/pull/10609/checks?check_run_id=6088325808 please ?

Once it is done I think we can merge it :)
Thanks.

[JokerQueue]

Done.

[Nexucis]

thanks @JokerQueue !