Change 'Failure' to 'Panic' in Bug Report Docs

Just saw this when looking at #19297 and couldn't find an issue/PR dealing with this. #18773 seems to have missed this file.

Compiler output is generated [here](https://github.com/rust-lang/rust/blob/770378a313a573776b16237a46b75bafa49072c1/src/librustc_trans/driver/mod.rs#L466).

cc @steveklabnik