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/ctx_manager.py

@@ -14,3 +14,7 @@ try:
         con.execute("insert into person(firstname) values (?)", ("Joe",))
 except sqlite3.IntegrityError:
     print("couldn't add Joe twice")
+
+# Connection object used as context manager only commits or rollbacks transactions,
+# so the connection object should be closed manually
+con.close()