Update installing-using-pip-and-virtual-environments.rst

[dharmatech]

Use Get-Command if on PowerShell

where in PowerShell is an alias for Where-Object.

If you use where in Powershell, no output is returned. Use Get-Command instead:

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📚 Documentation preview 📚: https://python-packaging-user-guide--1542.org.readthedocs.build/en/1542/