Fix README reference to stack_protocol

The parameter which we use to specify the protocol to connect to
the stack has always been named "stack_protocol" since its inception
in 5d8fa14.

For some reason, it's always been listed in the README as "protocol".

Fix that reference, and also move it from the list of required
parameters to that of optional ones (since it has a default, "ssh").

README.md

@@ -479,11 +479,12 @@ configured with the following attributes:
 * `stack_user_name`: The name of the user that the Xblock will use to connect
   to the environment, as specified in the orchestration template.
 
-* `protocol`: One of 'ssh', 'rdp', or 'vnc'.  This defines the protocol that
-  will be used to connect to the environment.  The default is 'ssh'.
-
 The following are optional:
 
+* `stack_protocol`: One of `ssh`, `rdp`, or `vnc`. This defines the
+  protocol that will be used to connect to the environment. The
+  default is `ssh`.
+
 * `stack_template_path`: The static asset path to the orchestration template,
   if not specified per provider below.