Correct Prompt Template

Based on the template shown in the [official model card](https://huggingface.co/timdettmers/guanaco-65b#usage) and the code from the [official playground space](https://huggingface.co/spaces/uwnlp/guanaco-playground-tgi/blob/main/app.py#L48-L49).

README.md

@@ -37,8 +37,8 @@ GGML files are for CPU + GPU inference using [llama.cpp](https://github.com/gger
 ## Prompt template
 
 ```
-USER: prompt
-ASSISTANT:
+### Human: prompt
+### Assistant:
 ```
 
 <!-- compatibility_ggml start -->