Clarify custom format example

[casper-hansen]

The current README seems a bit unclear on specifying a custom prompt format. In this PR, I modify the example to show a common scenario where you want a generation pair of prompt/completion.

I get rid of the YAML formatting of |- as it can quickly lead to unexpected behavior. One should just use a string, although it looks less fancy, it will be easier to ensure correctness.

    type:
      system_prompt: ""
      field_system: system
      format: "[INST] {instruction} [/INST]"
      no_input_format: "[INST] {instruction} [/INST]"

There are other more complicated formats that I choose to omit as you are better off using something that is not error-prone, especially when you are already using a custom dataset. Here is one example with Alpaca instruct, perhaps this can be included somewhere else in the docs:

    type:
      system_prompt: ""
      field_system: system
      format: "\n### Instruction: \n{instruction}\n### Output: \n"
      no_input_format: "\n### Instruction: \n{instruction}\n### Output: \n"

[winglian]

thanks!