fix: windows getting started guide for uniformity

This adds `ContainerAdministrator` as the default user to
guarantee a uniform experience on all the platforms.
The previous guide would fail on WS2022 but work
on WS2019 and Windows 11. The issue is being
investigated here moby#4731 

Signed-off-by: Anthony Nandaa <profnandaa@gmail.com>

docs/windows.md

@@ -89,6 +89,7 @@ Now that everything is setup, let's build a [simple _hello world_ image](https:/
     ```powershell
     Set-Content Dockerfile @"
     FROM mcr.microsoft.com/windows/nanoserver:ltsc2022
+    USER ContainerAdministrator
     COPY hello.txt C:/
     RUN echo "Goodbye!" >> hello.txt
     CMD ["cmd", "/C", "type C:\\hello.txt"]