[fix/doc] use http instead of git for checkout

Public users do not have git access to xgitlab, so they must clone the
repository using the read-only https url.

doc/tutorials/getting_started.rst

@@ -10,7 +10,7 @@ Download
 
 .. code-block:: console
   
-  $ git clone git@xgitlab.cels.anl.gov:argo/aml.git
+  $ git clone https://xgitlab.cels.anl.gov/argo/aml.git
   $ git checkout staging
 
 Requirements: