docs: use public git URL in the building Rudder section

The README currently recommends cloning the Rudder project using the
SSH git URL, which is not accessible to users outside of the CoreOS
GitHub organization.

Replace the SSH git URL with HTTPS to avoid confusing users and add
clarity to the build documentation.

README.md

@@ -24,7 +24,7 @@ overlay network:
 ## Building Rudder
 
 * Step 1: Make sure you have Linux headers installed on your machine. On Ubuntu, run ```sudo apt-get install linux-libc-dev```. On Fedora/Redhat, run ```sudo yum install kernel-headers```.
-* Step 2: Git clone the Rudder repo: ```git clone git@github.com:coreos/rudder.git```
+* Step 2: Git clone the Rudder repo: ```https://github.com/coreos/rudder.git```
 * Step 3: Run the build script: ```cd rudder; ./build```
 
 Alternatively, you can build rudder in a docker container with the following command. Replace $SRC with the absolute path to your rudder source code: