Fix comment of PQputCopyEnd()

The comment describing the error codes of this routine mentioned 0 as a
possible value, but this error code has never been used.

Author: Junwang Zhao
Reviewed-by: Aleksander Alekseev
Discussion: https://postgr.es/m/CAEG8a3Jt5KwMNr+_S6VN68rog4HeoG6ELvPQO8kZNQTeJeQ=rQ@mail.gmail.com

src/interfaces/libpq/fe-exec.c

@@ -2709,8 +2709,7 @@ PQputCopyData(PGconn *conn, const char *buffer, int nbytes)
  *
  * After calling this, use PQgetResult() to check command completion status.
  *
- * Returns 1 if successful, 0 if data could not be sent (only possible
- * in nonblock mode), or -1 if an error occurs.
+ * Returns 1 if successful, or -1 if an error occurs.
  */
 int
 PQputCopyEnd(PGconn *conn, const char *errormsg)