Fix instruction README for install by script

issue close MagicMirrorOrg#823

Author: roramirez

Diff for: README.md

@@ -31,7 +31,7 @@ Electron, the app wrapper around MagicMirror², only supports the Raspberry Pi 2
 
 Execute the following command on your Raspberry Pi to install MagicMirror²:
 ````
-curl -sL https://raw.githubusercontent.com/MichMich/MagicMirror/master/installers/raspberry.sh | bash
+bash -c "$(curl -sL https://raw.githubusercontent.com/MichMich/MagicMirror/master/installers/raspberry.sh)"
 ````
 
 ### Manual Installation