Fixed docs syntax for for-in loop in painless

docs/reference/modules/scripting/painless-syntax.asciidoc

@@ -207,7 +207,7 @@ In addition to Java's `enhanced for` loop, the `for in` syntax from groovy can a
 
 [source,painless]
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-for (item : list) {
+for (def item : list) {
   ...
 }
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