Fix document about StringQuery.

Original Pull Request #2202
Closes #2204

Author: matumu

src/main/asciidoc/reference/elasticsearch-operations.adoc

@@ -230,7 +230,7 @@ The following code shows a query that searches for persons having the first name
 [source,java]
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-Query query = new SearchQuery("{ \"match\": { \"firstname\": { \"query\": \"Jack\" } } } ");
+Query query = new StringQuery("{ \"match\": { \"firstname\": { \"query\": \"Jack\" } } } ");
 SearchHits<Person> searchHits = operations.search(query, Person.class);
 
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