server : update readme about token probs (#4777)

* updated server readme to reflect the gg/server-token-probs-4088 commit

added explanation for the API's completion result which now includes `completion_probabilities`. Also added a JSON schema that shows the type/structure of `completion_probabilities`.

* simplified the `completion_probabilities` JSON schema 

It's now easier to understand what the structure of `completion_probabilities` looks like.

* minor : fix trailing whitespace

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>