Typo in language reference.

I just found a typo in the language reference (docs/LangRef.html). The
section of Opaque Type contains "foward declared", which should of
course be "forward declared".

Greetings,
Kevin André

Fixed here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20071008/054501.html

— Gordon