Uses string literals instead of constants

more clear about used options in the example

Author: lajosthiel

components/serializer.rst

@@ -900,8 +900,8 @@ Option                  Description                                           De
 ``csv_headers``         Sets the order of the header and data columns
                         E.g.: $this->serializer->serialize(
                             ['c' => 3,'a' => 1,'b' => 2],
-                            CsvEncoder::FORMAT,
-                            [CsvEncoder::HEADERS_KEY => ['a', 'b', 'c']]);
+                            'csv',
+                            ['csv_headers' => ['a', 'b', 'c']]);
                         returns
                             a,b,c
                             1,2,3                                             ``[]``, inferred from input data's keys