Edit getindex Operator.tid op-purpose (#2837)

Other `op-` fields should probably also be added.

editions/tw5.com/tiddlers/filters/getindex.tid

@@ -3,7 +3,7 @@ modified: 20150203140000000
 tags: [[Filter Operators]] [[Field Operators]]
 title: getindex Operator
 caption: getindex
-purpose: select all values of a data property in the input titles
+op-purpose: select all values of a data property in the input titles
 
 <$macrocall $name=".operator-def"
 input="a [[selection of titles|Title Selection]]"
@@ -14,4 +14,4 @@ output="the values of property <<.place P>> in each of the input titles"
 
 Each input title is processed in turn, and is ignored if it does not denote a [[data tiddler|DataTiddlers]]. If the tiddler contains property <<.place P>>, the value of that property is [[dominantly appended|Dominant Append]] to the output.
 
-<<.operator-examples "getindex">>
+<<.operator-examples "getindex">>