Update flan-t5 original model link (#20897)

Update flan-t5.mdx

docs/source/en/model_doc/flan-t5.mdx

@@ -46,4 +46,4 @@ Google has released the following variants:
 
 One can refer to [T5's documentation page](t5) for all tips, code examples and notebooks. As well as the FLAN-T5 model card for more details regarding training and evaluation of the model.
 
-The original checkpoints can be found [here](https://github.com/google-research/t5x/blob/main/docs/models.md#mixture-of-experts-moe-checkpoints).
+The original checkpoints can be found [here](https://github.com/google-research/t5x/blob/main/docs/models.md#flan-t5-checkpoints).
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