[Flux] Update integration test badge in README.md (#2019)

Fixes the badge in the `README.md` file

Author: rm-wu

torchtitan/models/flux/README.md

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 # FLUX model in torchtitan
 
-[![integration tests](https://github.com/pytorch/torchtitan/actions/workflows/integration_test_8gpu_flux.yaml/badge.svg?branch=main)](https://github.com/pytorch/torchtitan/actions/workflows/integration_test_8gpu_flux.yaml/badge.svg?branch=main)
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 ## Overview
 This directory contains the implementation of the [FLUX](https://github.com/black-forest-labs/flux/tree/main) model in torchtitan. In torchtitan, we showcase the pre-training process of text-to-image part of the FLUX model.