Fix link to speech encoder decoder model in speech recognition readme
# What does this PR do?

Current README documentation aims to `https://huggingface.co/docs/transformers/main/en/model_doc/speechencoderdecoder#speech-encoder-decoder-models`, which redirects to a 404 Not found. The actual link seems to be `https://huggingface.co/docs/transformers/main/en/model_doc/speech-encoder-decoder#speech-encoder-decoder-models`
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20633). All of your documentation changes will be reflected on that endpoint.