readme: add language property to code blocks (#287)

README.md

@@ -41,32 +41,33 @@ the installers for Debian and RPM-based distributions.
 
 To setup the package repository, run these commands:
 
-```
+```sh
 $ wget -qO - https://packagecloud.io/shiftkey/desktop/gpgkey | sudo apt-key add -
 $ sudo sh -c 'echo "deb [arch=amd64] https://packagecloud.io/shiftkey/desktop/any/ any main" > /etc/apt/sources.list.d/packagecloud-shiftky-desktop.list'
 $ sudo apt-get update
 ```
 
 Then install GitHub Desktop:
 
-```
+```sh
 $ sudo apt install github-desktop
 ```
 
 ### Red Hat/CentOS/Fedora distributions
 
 To setup the package repository, run these commands:
 
-```
+```sh
 $ sudo rpm --import https://packagecloud.io/shiftkey/desktop/gpgkey
 $ sudo sh -c 'echo -e "[shiftkey]\nname=GitHub Desktop\nbaseurl=https://packagecloud.io/shiftkey/desktop/el/7/\$basearch\nenabled=1\ngpgcheck=0\nrepo_gpgcheck=1\ngpgkey=https://packagecloud.io/shiftkey/desktop/gpgkey" > /etc/yum.repos.d/shiftkey-desktop.repo'
 ```
 
 Then install GitHub Desktop:
 
-```
+```sh
 # if yum is your package manager
 $ sudo yum install github-desktop
+
 # if dnf is your package manager
 $ sudo dnf install github-desktop
 ```