Fix outdated comment of COPY in gram.y.

Author: ChangAo Chen <cca5507@qq.com>
Discussion: https://postgr.es/m/tencent_392C0E92EC52432D0A336B9D52E66426F009@qq.com

src/backend/parser/gram.y

@@ -3408,7 +3408,7 @@ ClosePortalStmt:
  *				COPY ( query ) TO file	[WITH] [(options)]
  *
  *				where 'query' can be one of:
- *				{ SELECT | UPDATE | INSERT | DELETE }
+ *				{ SELECT | UPDATE | INSERT | DELETE | MERGE }
  *
  *				and 'file' can be one of:
  *				{ PROGRAM 'command' | STDIN | STDOUT | 'filename' }