doc/_ext: print "default" using "literal" filter

there is chance that the default value of an option is empty, before
this change the rendered result would be "````", and sphinx is confused
at seeing this:

Unexpected section title or transition.

as it takes it as a notion of section title. after this change,
the "literal" filter is always applied to the default value, so an empty
string is rendered like "<empty string>" instead.

Signed-off-by: Kefu Chai <kchai@redhat.com>

doc/_ext/ceph_confval.py

@@ -41,7 +41,7 @@ TEMPLATE = '''
   {%- elif opt.type == 'bool' %}
    :default: ``{{ default | string | lower }}``
   {%- else %}
-   :default: ``{{ default }}``
+   :default: {{ default | literal }}
   {%- endif -%}
 {%- endif %}
 {%- if opt.enum_values %}