Fix doc issue for "compute agent list" command

"--hypervisor" is a optional argument but in the doc it
was wrong format, this patch modify it.

Change-Id: I527feff73eca5834b2fee4efb2462562581210ed

Author: Huanxuan Ao

doc/source/command-objects/compute-agent.rst

@@ -66,10 +66,9 @@ List compute agents
 
     os compute agent list [--hypervisor <hypervisor>]
 
-.. _compute_agent-list:
-.. describe:: --hypervisor <hypervisor>
+.. option:: --hypervisor <hypervisor>
 
-    Optional type of hypervisor
+    Type of hypervisor
 
 compute agent set
 -----------------