fix sphinx-lint issues in api guide

as with the previous commit this change is simply correcting the
usage of backticks for inline literals

Change-Id: Icbfd168266dc1348ee15f7347ed673d220989ceb

api-guide/source/server_concepts.rst

@@ -95,8 +95,8 @@ are exposed to administrators:
   Refer to :nova-doc:`VM States <reference/vm-states.html>`.
 
 - task_state represents what is happening to the instance at the
-  current moment. These tasks can be generic, such as `spawning`, or specific,
-  such as `block_device_mapping`. These task states allow for a better view into
+  current moment. These tasks can be generic, such as ``spawning``, or specific,
+  such as ``block_device_mapping``. These task states allow for a better view into
   what a server is doing.
 
 Server creation