fix example extension migration template.

lnbits/extensions/example/migrations.py

@@ -1,11 +1,10 @@
 # async def m001_initial(db):
-
 #    await db.execute(
-#        """
+#        f"""
 #        CREATE TABLE example.example (
 #            id TEXT PRIMARY KEY,
 #            wallet TEXT NOT NULL,
-#            time TIMESTAMP NOT NULL DEFAULT """ + db.timestamp_now + """
+#            time TIMESTAMP NOT NULL DEFAULT {db.timestamp_now}
 #        );
 #    """
 #    )