Update README.md (Powershell Install One-Liner) (#1003)

A tidy one liner for the powershell install command. 
Much like the unix one 😄

README.md

@@ -44,14 +44,8 @@ file as suggested [here][auto].
 ###### Windows (PowerShell)
 
 ```powershell
-md ~\vimfiles\autoload
-$uri = 'https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
-(New-Object Net.WebClient).DownloadFile(
-  $uri,
-  $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath(
-    "~\vimfiles\autoload\plug.vim"
-  )
-)
+iwr -useb https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim |`
+    ni $HOME/vimfiles/autoload/plug.vim -Force
 ```
 
 #### Neovim
@@ -73,14 +67,8 @@ curl -fLo ~/.var/app/io.neovim.nvim/data/nvim/site/autoload/plug.vim \
 ###### Windows (PowerShell)
 
 ```powershell
-md ~\AppData\Local\nvim-data\site\autoload
-$uri = 'https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
-(New-Object Net.WebClient).DownloadFile(
-  $uri,
-  $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath(
-    "~\AppData\Local\nvim-data\site\autoload\plug.vim"
-  )
-)
+iwr -useb https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim |`
+    ni "$env:LOCALAPPDATA/nvim-data/site/autoload/plug.vim" -Force
 ```
 
 ### Getting Help