Update field-names-field.asciidoc
fix typo in field name
@@ -6,7 +6,7 @@ contains any value other than `null`. This field is used by the
<<query-dsl-exists-query,`exists`>> query to find documents that
either have or don't have any non-+null+ value for a particular field.
-The value of the `_field_name` field is accessible in queries:
+The value of the `_field_names` field is accessible in queries:
[source,js]
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