window function: function name of error message should be lowercase

[SunRunAway]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

SELECT NTH_VALUE(fieldA, 0) RESPECT NULLS OVER ( ORDER BY fieldB DESC, fieldA ASC ) AS 'nth_value', fieldA, fieldB FROM ( SELECT `col_varchar` AS fieldA, `col_time_key` AS fieldB FROM `table60_int_autoinc` ) as t WINDOW w1 AS ( ORDER BY fieldA DESC, fieldB ROWS BETWEEN 0 FOLLOWING AND CURRENT ROW);

2. What did you expect to see?

ERROR 1210 (HY000) at line 1: Incorrect arguments to nth_value

3. What did you see instead?

ERROR 1210 (HY000) at line 1: Incorrect arguments to NTH_VALUE

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

[tcmichael]

/assign