update docs to add an example of the prompting format, as it is imple…

[flu0r1ne]

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Why are these changes needed?

The example prompt template in the docs does not reflect the prompt template used when training Vicuna. Other implementers may be mislead and this could lead to sub-optimal results.

Updated the documentation accordingly.

Related issue number (if applicable)

#1673

Checks

• [x] I've run format.sh to lint the changes in this PR. (docs update)
• [x] I've included any doc changes needed.
• [x] I've made sure the relevant tests are passing (if applicable).