debug/dwarf: update PDF link.

Fixes #881.

R=iant
CC=golang-dev
https://golang.org/cl/1696044

src/pkg/debug/dwarf/open.go

@@ -4,7 +4,7 @@
 
 // This package provides access to DWARF debugging information
 // loaded from executable files, as defined in the DWARF 2.0 Standard
-// at http://dwarfstd.org/dwarf-2.0.0.pdf.
+// at http://dwarfstd.org/doc/dwarf-2.0.0.pdf
 package dwarf
 
 import (