Amazon Bedrock AgentCore Samples - Improve README.md installation process

**Describe the improvement request**
In the README.md, in the installation instructions, `source .venv/bin/activate` is only applicable for Linux environments.

**What are your suggestions?**
Add a tiny note that says: For windows users run `.venv/scripts/activate`

**Describe alternatives you've considered**
n/a

**Screenshots**
n/a

**Additional context**
n/a