Clarify Windows install command in README.md (#1751)

Specify that the command to run for Windows is for powershell

## Summary

Clarifies the usage of the command to run to install uv on Windows
(powershell only)

## Test Plan

It wasn't 😈

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Co-authored-by: konsti <konstin@mailbox.org>

README.md

@@ -37,7 +37,7 @@ Install uv with our standalone installers, or from [PyPI](https://pypi.org/proje
 # On macOS and Linux.
 curl -LsSf https://astral.sh/uv/install.sh | sh
 
-# On Windows.
+# On Windows (with PowerShell).
 irm https://astral.sh/uv/install.ps1 | iex
 
 # With pip.