Contributing.md Minor Suggestion

[tgkolda]

I'm creating a fresh install of pyttb. I use a local conda Python environment. The default instructions in CONTRIBUTING.md did not work for me. Instead, I had to prepend the pip commands with python -m. I asked Copilot why that was, and it explained: "it ensures that the pip version being used is the one associated with the specific Python interpreter you're invoking". So, I wonder if it would be worthwhile to change the two lines as follows:

python -m pip install -e ".[dev]"

and

python -m pip install -e ".[dev,doc]"