Update example (#7788)

Author: FedericoCeratto

lib/impure/db_sqlite.nim

@@ -31,7 +31,7 @@
 ##
 ## .. code-block:: Nim
 ## import db_sqlite
-## let db = open("localhost", "user", "password", "dbname")
+## let db = open("mytest.db", nil, nil, nil) # user, password, database name can be nil
 ## db.close()
 ##
 ## Creating a table