Update Readme for reusing python env (openvinotoolkit#318)

update readme about how to upgrade python packages when reusing existing
python env.

![image](https://github.com/openvinotoolkit/openvino.genai/assets/164128732/46a0ea71-79f1-49a9-a339-db3dc5ff45a0)

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Co-authored-by: Chen Peter <peter.chen@intel.com>

llm_bench/python/README.md

@@ -11,10 +11,12 @@ pytorch and openvino models, using almost the same code and precollected models.
 ``` bash
 python3 -m venv python-env
 source python-env/bin/activate
-pip install update --upgrade
+pip install --upgrade pip
 pip install -r requirements.txt
 ```
-> Note: For llama models, ensure to have transformers<4.38.
+> Note:
+> If you are using an existing python environment, recommend following command to use all the dependencies with latest versions:  
+> pip install -U --upgrade-strategy eager -r requirements.txt
 
 ### 2. Convert a model to OpenVINO IR