update apt-key

I think 'hkp://keyserver.ubuntu.com:80' would be better.

Author: JaymzZh

docs/getting-started/installing-core-linux.rst

@@ -30,7 +30,7 @@ You also need a latest version of Mono, which is required for DNX tooling. This
 
 .. code-block:: console
 
-    sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
+    sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
     echo "deb http://download.mono-project.com/repo/debian wheezy main" | sudo tee /etc/apt/sources.list.d/mono-xamarin.list
     sudo apt-get update
     sudo apt-get install mono-complete