Fixed the string() code literal in the java client index api doc.

docs/java-api/index_.asciidoc

@@ -114,7 +114,7 @@ Note that you can also add arrays with `startArray(String)` and
 other XContentBuilder objects.
 
 If you need to see the generated JSON content, you can use the
-@string()@method.
+`string()` method.
 
 [source,java]
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