[DOC] Fixed flags example incorrect syntax

docs/reference/query-dsl/queries/regexp-query.asciidoc

@@ -39,8 +39,10 @@ You can also use special flags
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 {
     "regexp":{
-        "name.first": "s.*y",
-        "flags" : "INTERSECTION|COMPLEMENT|EMPTY"
+        "name.first": {
+            "value": "s.*y",
+            "flags" : "INTERSECTION|COMPLEMENT|EMPTY"
+        }
     }
 }
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