docs: update readme with correct venv name

[jerrylin3321]

I was trying to get my dev environment set up, and it wasn't pulling the right version of Python

Turns out this is because the instructions expect the env virtual environment

warning: `VIRTUAL_ENV=env` does not match the project environment path `.venv` and will be ignored; 
use `--active` to target the active environment instead

[greptile-apps]

Greptile Summary

This PR fixes a documentation inconsistency in the README.md file regarding virtual environment setup for the PostHog Python SDK development environment. The change updates the uv venv command to explicitly specify uv venv env to create a virtual environment named 'env' instead of the default '.venv'.

The issue arose because uv venv without arguments creates a default .venv directory, but the documentation throughout the README consistently references an 'env' directory - in the activation command source env/bin/activate and in the step-by-step instructions. This mismatch caused uv to display warnings about VIRTUAL_ENV=env not matching the project environment path .venv.

By explicitly naming the virtual environment 'env', the code block now aligns with the existing step-by-step instructions and ensures developers can follow the setup process without encountering path mismatch warnings. This is a simple documentation fix that improves the developer experience when setting up the local development environment.

Confidence score: 5/5

• This is an extremely safe documentation fix that resolves a real development setup issue
• The change is minimal, well-justified, and aligns with existing documentation patterns
• No files require additional attention - this is a straightforward README correction

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