update comment

Author: giwa

test/test_value_converter_factory.rb

@@ -248,7 +248,7 @@ def test_date
           assert_equal nil, converter.call(nil)
           assert_equal "2016-02-26", converter.call("2016/02/26")
 
-          # Users must care of BQ timestamp format by themselves with no timestamp_format
+          # Users must care of BQ date format by themselves with no timestamp_format
           converter = ValueConverterFactory.new(SCHEMA_TYPE, 'DATE').create_converter
           assert_equal nil, converter.call(nil)
           assert_equal "2016-02-26", converter.call("2016-02-26")