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/adapter_datetime.py

@@ -13,3 +13,5 @@ cur = con.cursor()
 now = datetime.datetime.now()
 cur.execute("select ?", (now,))
 print(cur.fetchone()[0])
+
+con.close()