docs: Fix syntax error in flask migration doc (#4061)

Author: Piechutowski

docs/migration/flask.rst

@@ -278,7 +278,7 @@ in Litestar.
    from litestar import Litestar
    from litestar.static_files import create_static_files_router
 
-    app = Litestar route_handlers=[
+    app = Litestar(route_handlers=[
         create_static_files_router(path="/static", directories=["assets"]),
     ])