Fix typo

Documentation/Variable.rst

@@ -462,7 +462,7 @@ These variables execute a qfqFunction::
   {{getFullname(pId) => fullname AS function}}
   {{SELECT '{{fullname:RE}}'}}
 
-Usually it is preferred to get the output from a function without using the variable {{_output:RE}}::
+Usually it is preferred to get the output from a function without using the variable {{output:RE}}::
 
   // Report with qfqFunction. Subheader: getFullname
   render = api