Changed apt-get to apt in the debian installation section of the read…

…me file (#656)

README.md

@@ -44,7 +44,7 @@ To setup the package repository, run these commands:
 ```sh
 $ wget -qO - https://packagecloud.io/shiftkey/desktop/gpgkey | sudo tee /etc/apt/trusted.gpg.d/shiftkey-desktop.asc > /dev/null
 $ sudo sh -c 'echo "deb [arch=amd64] https://packagecloud.io/shiftkey/desktop/any/ any main" > /etc/apt/sources.list.d/packagecloud-shiftkey-desktop.list'
-$ sudo apt-get update
+$ sudo apt update
 ```
 
 Then install GitHub Desktop: