Updates section about Python in Windows Build Instructions

When following https://chromium.googlesource.com/chromium/src/+/HEAD/docs/windows_build_instructions.md on a non-corp Windows device I found that

> where python

Did not provide any output. The correct test of a valid python path
should be

> where python3

and this is now also added to the documentation.

Bug: None
Change-Id: Iaf67f83e3ba2507aa68693c365fcfa22b77ad1c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425018


Commit-Queue: Henrik Andreasson <henrika@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283130}