Update current-timestamp.md

[dsporov]

It's not allowed to invoke CURRENT_TIMESTAMP in the way it's specified in the docs: SELECT CURRENT_TIMESTAMP();, it just doesn't work:

Invalid input error: SQL query not valid.
SQL query:
==========
select CURRENT_TIMESTAMP()
                        ^=== ERROR HERE!
==========
Error line: 0
Error column: 24
Error message: syntax error, unexpected '(', expecting end of file

The following syntax should be used instead: SELECT CURRENT_TIMESTAMP;

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