Update pip command in installation.rst

[ejhw]

Fixes # .

Pull Request Description

pip install -e . gave me "-bash: pip: command not found" so changing to python -m pip install -e . which appears to be best practice (https://stackoverflow.com/a/9780772)

Then I got
"Could not install packages due to an EnvironmentError: [Errno 13] Permission denied: '/Library/Python/3.7'
Consider using the --user option or check the permissions."
so I added "--user" though alternatively we could say to only do that if you get the error.

Checklist

Not all may apply

• Code changes (must work)
• Tests exercising your feature/bug fix (check coverage report on CircleCI build -> Artifacts)
• All other unit tests passing
• Update validation for project config yaml file changes
• Update existing documentation
• Run a small batch run to make sure it all works (local is fine, unless an Eagle specific feature)
• Add to the changelog_dev.rst file and propose migration text in the pull request