Update PSGet install command

Author: staxmanade

README.md

@@ -24,7 +24,7 @@ Installation
 First, you need to download the module.  Using PsGet (http://psget.net, very easy to install), you can run:
 
 ```
-psget PSReadline
+install-module PSReadline
 ```
 
 Alternatively, download the file https://github.com/lzybkr/PSReadLine/raw/master/PSReadline.zip and extract the contents into your `C:\Users\[User]\Documents\WindowsPowerShell\modules\PSReadline` folder. (You may have to create these directories if they don't exist)