Update shell-commands.rst to use %USERPROFILE% in Windows PATH (#371)

core/installation/shell-commands.rst

@@ -91,5 +91,4 @@ Windows
 Please read one of these instructions `How do I set or change the PATH system variable? <https://www.google.com.ua/search?q=how+do+i+set+or+change+the+path+system+variable>`_
 
 You need to edit the system environment variable called ``Path`` and append
-``C:\Users\UserName\.platformio\penv\Scripts\`` path in the beginning of a
-list (please replace ``UserName`` with your account name).
+``%USERPROFILE%\.platformio\penv\Scripts\`` path in the beginning of the list.