tf_bart typo - self.self.activation_dropout
# What does this PR do?
Fix one line typo in `modeling_tf_bart` :

`self.self.activation_dropout` -> `self.activation_dropout`

BTW, there's no error in forward pass. 
I met the error when I played around with `model.fit()` :)

## Who can review ?
@sshleifer 
LGTM, cc @LysandreJik. I won't merge on my own anymore.