Change PROCEDURE to FUNCTION in CREATE OPERATOR syntax

Change PROCEDURE to FUNCTION in CREATE OPERATOR syntax

Since procedures are now a different thing from functions, change the
CREATE OPERATOR syntax to use FUNCTION in the clause that specifies the
function.  PROCEDURE is still accepted for compatibility.

Reported-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Jonathan S. Katz <jonathan.katz@excoventures.com>