Update start-nodes.md with working sudo syntax

This fixes a line that will error out with permission denied due to the appending / redirect of output not having sudo, instead running it as an encapsulated sudo command

 Author:    Kevin <kevin@kiva.org>

Signed-off-by: Kevin OBrien <kevin@kiva.org>

docs/start-nodes.md

@@ -7,7 +7,7 @@ In order to run your own Network, you need to do the following for each Node:
     - A recommended way for ubuntu is installing from deb packages
     ```
     sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 68DB5E88
-    sudo echo "deb https://repo.sovrin.org/deb xenial stable" >> /etc/apt/sources.list
+    sudo bash -c 'echo "deb https://repo.sovrin.org/deb xenial stable" >> /etc/apt/sources.list'
     sudo apt-get update
     sudo apt-get install indy-node
     ```