Fix access variable name programmatically faq docs (#64041)

* Add an example of replacing dashes with underscores in interface names for IPv4

docs/docsite/rst/reference_appendices/faq.rst

@@ -379,9 +379,15 @@ via a role parameter or other input.  Variable names can be built by adding stri
 
     {{ hostvars[inventory_hostname]['ansible_' + which_interface]['ipv4']['address'] }}
 
-The trick about going through hostvars is necessary because it's a dictionary of the entire namespace of variables.  'inventory_hostname'
+The trick about going through hostvars is necessary because it's a dictionary of the entire namespace of variables.  ``inventory_hostname``
 is a magic variable that indicates the current host you are looping over in the host loop.
 
+In the example above, if your interface names have dashes, you must replace them with underscores:
+
+.. code-block:: jinja
+
+    {{ hostvars[inventory_hostname]['ansible_' + which_interface | replace('_', '-') ]['ipv4']['address'] }}
+
 Also see dynamic_variables_.