bpo-34580: Update sqlite3 examples to call close() explicitly (GH-9079)

The sqlit3.Connection object doesn't call its close() method when it's used
as a context manager.

Doc/includes/sqlite3/shortcut_methods.py

@@ -18,3 +18,7 @@ for row in con.execute("select firstname, lastname from person"):
     print(row)
 
 print("I just deleted", con.execute("delete from person").rowcount, "rows")
+
+# close is not a shortcut method and it's not called automatically,
+# so the connection object should be closed manually
+con.close()