Fix comment in modeling_t5.py
# What does this PR do?
This PR completes an incomplete comment in the modeling_t5.py file. 

`# ourselves in which case we just need to make it broadcastable to all heads.`
to 
```
# We can provide a self-attention mask of dimensions [batch_size, from_seq_length, to_seq_length] 
# ourselves in which case we just need to make it broadcastable to all heads.
```
