database/sql: add "defer rows.Close()" to the example code.

Strictly speaking, it's not necessary in example_test.go, as the
Rows.Close docs say that "If Next returns false, the Rows are closed
automatically". However, if the for loop breaks or returns early, it's
not obvious that you'll leak unless you explicitly call Rows.Close.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews, rsc
https://golang.org/cl/79330043

src/pkg/database/sql/example_test.go

@@ -18,6 +18,7 @@ func ExampleDB_Query() {
 	if err != nil {
 		log.Fatal(err)
 	}
+	defer rows.Close()
 	for rows.Next() {
 		var name string
 		if err := rows.Scan(&name); err != nil {

src/pkg/database/sql/sql.go

@@ -1494,6 +1494,7 @@ func (s *Stmt) finalClose() error {
 //
 //     rows, err := db.Query("SELECT ...")
 //     ...
+//     defer rows.Close()
 //     for rows.Next() {
 //         var id int
 //         var name string