Standardize system prompt format for AlpacaPrompter (instruct case)

[sadaisystems]

Description

I changed system prompt format for prompt template of Alpaca Prompter: removed ###System\n before system message, so it aligns with commonly used Alpaca template.

Motivation and Context

After a short discussion of some problems related to Alpaca template, we found out, that in axolotl implementation system prompt starts with ### System, which is however not a standard way to to this. Link to discussion message - link

Link to an example of commonly used alpaca prompt template in alpaca-lora repo - link

This change might save a lot of time for beginners that are not aware of this unconventional prompt change that was implemented in axolotl.

How has this been tested?

This change shoud not influence framework functionality, it just adjusts prompt template.

[winglian]

thanks!