updateing filtering.asciidoc to also use 'node.attr' namespace

docs/reference/index-modules/allocation/filtering.asciidoc

@@ -14,7 +14,7 @@ attribute as follows:
 
 [source,sh]
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-bin/elasticsearch -Ees.node.rack=rack1 -Ees.node.size=big  <1>
+bin/elasticsearch -Ees.node.attr.rack=rack1 -Ees.node.attr.size=big  <1>
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 <1> These attribute settings can also be specified in the `elasticsearch.yml` config file.