update FIM tokens and examples

based on https://huggingface.co/bigcode/tiny_starcoder_py/discussions/2#64734c6e16ef9c21dba65637

README.md

@@ -61,7 +61,7 @@ print(tokenizer.decode(outputs[0]))
 Fill-in-the-middle uses special tokens to identify the prefix/middle/suffix part of the input and output:
 
 ```python
-input_text = "<fim-prefix>def print_hello_world():\n    <fim-suffix>\n    print('Hello world!')<fim-middle>"
+input_text = "<fim_prefix>def print_one_two_three():\n    print('one')\n    <fim_suffix>\n    print('three')<fim_middle>"
 inputs = tokenizer.encode(input_text, return_tensors="pt").to(device)
 outputs = model.generate(inputs)
 print(tokenizer.decode(outputs[0]))