install: fix ubuntu docs.

obs provides ubuntu packages by http not https.

Fixes: #81

install/ubuntu-installation-guide.md

@@ -23,8 +23,8 @@ For more information on installing Docker please refer to the
 2. Install the Kata Containers components with the following commands:
 
 ```
-$ sudo sh -c "echo 'deb https://download.opensuse.org/repositories/home:/katacontainers:/release/xUbuntu_$(lsb_release -rs)/ /' >> /etc/apt/sources.list.d/kata-containers.list"
-$ curl -sL  https://download.opensuse.org/repositories/home:/katacontainers:/release/xUbuntu_$(lsb_release -rs)/Release.key | sudo apt-key add -
+$ sudo sh -c "echo 'deb http://download.opensuse.org/repositories/home:/katacontainers:/release/xUbuntu_$(lsb_release -rs)/ /' >> /etc/apt/sources.list.d/kata-containers.list"
+$ curl -sL http://download.opensuse.org/repositories/home:/katacontainers:/release/xUbuntu_$(lsb_release -rs)/Release.key | sudo apt-key add -
 $ sudo -E apt-get update
 $ sudo -E apt-get -y install kata-runtime kata-proxy kata-shim
 ```