[docs] Use dbgs() instead of errs() for DEBUG()

The examples in llvm/Support/Debug.h use `DEBUG(dbgs() << ...)` instead
of `errs()`, so the examples in the Programmer's Manual should match
that.

Patch by: Moritz Sichert <moritz.sichert@googlemail.com>

Differential revision: https://reviews.llvm.org/D41170

llvm-svn: 321444