Fix copy/paste mistake in comment

Backpatch to 17

Author: Yugo NAGATA <nagata@sraoss.co.jp>
Discussion: https://postgr.es/m/20240704134638.355ad44a445fa1e764a220cd@sranhm.sraoss.co.jp

Author: alvherre

src/interfaces/libpq/fe-cancel.c

@@ -299,7 +299,8 @@ PQcancelSocket(const PGcancelConn *cancelConn)
 /*
  *		PQcancelErrorMessage
  *
- * Get the socket of the cancel connection.
+ * Returns the error message most recently generated by an operation on the
+ * cancel connection.
  */
 char *
 PQcancelErrorMessage(const PGcancelConn *cancelConn)