rabbit: use double backticks

Problem: code snippets in the rabbit docs are formatted incorrectly
because they only use single backticks.

Change to double backticks.

tutorials/lab/rabbit_config.rst

@@ -58,12 +58,12 @@ Flux's interactions with the rabbits.
 
 **mapping** (string)
   (required) Path to rabbitmapping file for the cluster, as generated by
-  `flux rabbitmapping`.
+  ``flux rabbitmapping``.
 
 **kubeconfig** (string)
   (optional) Path to kubeconfig file for Flux to use, ideally with restricted permissions.
-  This can be left undefined if the file is placed at the path `~flux/.kube/config`
-  (assuming the `flux` user is the instance owner).
+  This can be left undefined if the file is placed at the path ``~flux/.kube/config``
+  (assuming the ``flux`` user is the instance owner).
 
 **tc_timeout** (integer)
   (optonal) Time in seconds to tolerate a workflow stuck in TransientCondition state
@@ -74,21 +74,21 @@ Flux's interactions with the rabbits.
   with their rabbit. Defaults to true.
 
 **save_datamovements** (integer)
-  (optional) Number of `nnfdatamovement` resources to save to jobs' KVS, may be useful for
+  (optional) Number of ``nnfdatamovement`` resources to save to jobs' KVS, may be useful for
   debugging but too many may degrade performance. Defaults to 0.
 
 **restrict_persistent_creation** (boolean)
   (optional) Restrict the creation of persistent file systems to the instance owner
-  (in most cases the `flux` user).
+  (in most cases the ``flux`` user).
 
 **policy.maximums** (table)
   (optional) The maximum filesystem capacity per node, in GiB, that users may
   request. Leave undefined for no limit. See below for an example.
 
 **presets** (table)
   (optional) Defines preset #DW strings. May potentially save users time and energy,
-  allowing them to run, for instance, `flux alloc -N1 -S dw=NAME` rather than
-  `flux alloc -N1 -S "dw=#DW jobdw ..."` See below for an example.
+  allowing them to run, for instance, ``flux alloc -N1 -S dw=NAME`` rather than
+  ``flux alloc -N1 -S "dw=#DW jobdw ..."`` See below for an example.
 
 
 Example