Revise Windows installation command

ps is the process listing command (installed as part of git bash); it does not invoke PowerShell. The command given in the readme actually overwrites install.ps1 with the output of the process list. Invoking powershell -file install.ps1 will do the job.

Author: Jerry Kindall

README.rst

@@ -59,7 +59,7 @@ the directory to the current user ``PATH``.
 
 ::
 
-    PS > ./install.ps1
+    powershell -file install.ps1
 
 Homebrew (for macOS users)
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