fix some docstring in encoder-decoder models
# What does this PR do?

Fix a few places in some encoder-decoder docstrings. For example,

`:class:~transformers.EncoderDecoder is a generic ...` -> `:class:~transformers.SpeechEncoderDecoderModel is a generic ... `
in `modeling_speech_encoder_decoder.py`.

## Who can review?

@patrickvonplaten 
