ox-confluence.el: Fix error in `org-confluence-fixed-width`

* contrib/lisp/ox-confluence.el (org-confluence-fixed-width): retrieve
  value of org element "fixed-width" instead of using "content", which
  is empty.

contrib/lisp/ox-confluence.el

@@ -93,7 +93,8 @@
             (org-trim contents))))
 
 (defun org-confluence-fixed-width (fixed-width contents info)
-  (format "\{\{%s\}\}" contents))
+  (format "\{\{%s\}\}"
+         (org-trim (org-element-property :value fixed-width))))
 
 (defun org-confluence-verbatim (verbatim contents info)
   (format "\{\{%s\}\}" (org-element-property :value verbatim)))