Fix TFSegformerForSemanticSegmentation doctest
# What does this PR do?

Fix `TFSegformerForSemanticSegmentation` doctest.
With @amyeroberts uploading `nvidia/mit-b0`, all doctests for TF Segformer pass now.

(I was waiting for a TF checkpoint and not fixed this one in the previous PR. This model (SemanticSegmentation) already had a TF checkpoint however)
_The documentation is not available anymore as the PR was closed or merged._