Re: Exception Hierarchy Docs

[euant]

Looks like it's been fixed on the devel branch, so should be fixed next time 
the docs are regenerated (next release): 
[https://github.com/nim-lang/Nim/blob/devel/lib/system.nim#L424](https://github.com/nim-lang/Nim/blob/devel/lib/system.nim#L424)