Fix mismatched symbol in MySQLLexer

[evan-choi]

TIMESTAMP_ADD -> TIMESTAMPADD
TIMESTAMP_DIFF -> TIMESTAMPDIFF

MySQL Workbench

mysql-workbench/library/parsers/grammars/MySQLLexer.g4

Line 866 in 2f35f90

TIMESTAMP_ADD_SYMBOL: T I M E S T A M P '_' A D D;

mysql-workbench/library/parsers/grammars/MySQLLexer.g4

Line 867 in 2f35f90

TIMESTAMP_DIFF_SYMBOL: T I M E S T A M P '_' D I F F;

MySQL Server

https://github.com/mysql/mysql-server/blob/8.0/sql/lex.h#L746
https://github.com/mysql/mysql-server/blob/8.0/sql/lex.h#L747

[mysql-oca-bot]

Hi, thank you for submitting this pull request. In order to consider your code we need you to sign the Oracle Contribution Agreement (OCA). Please review the details and follow the instructions at https://oca.opensource.oracle.com/
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Thanks

[evan-choi]

Submitted OCA

[mysql-oca-bot]

Hi, thank you for your contribution. Please confirm this code is submitted under the terms of the OCA (Oracle's Contribution Agreement) you have previously signed by cutting and pasting the following text as a comment:
"I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it."
Thanks

[evan-choi]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

[mysql-oca-bot]

Hi, thank you for your contribution. Your code has been assigned to an internal queue. Please follow
bug http://bugs.mysql.com/bug.php?id=106380 for updates.
Thanks