Fix malformed JSON in Delete API example (#21168)

Obviously, there should be

    "result": "deleted"

instead of

    "result: deleted"

docs/reference/docs/delete.asciidoc

@@ -26,7 +26,7 @@ The result of the above delete operation is:
     "_type" : "tweet",
     "_id" : "1",
     "_version" : 2,
-    "result: deleted"
+    "result": "deleted"
 }
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