Clarify that the installation script for Windows requires PowerShell

https://github.com/junegunn/vim-plug/pull/309#issuecomment-155259326

I take back my words :(
I thought PowerShell means the default Windows shell.

README.md

@@ -41,7 +41,7 @@ curl -fLo ~/.config/nvim/autoload/plug.vim --create-dirs \
     https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
 ```
 
-###### Windows
+###### Windows (PowerShell)
 
 ```powershell
 md ~\vimfiles\autoload