doc: fix source link in gdb docs

Fixes #12059.

Change-Id: Ib5caf8133cd3ed888f9102dfbfeca11c506f3b5b
Reviewed-on: https://go-review.googlesource.com/18337Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/debugging_with_gdb.html

@@ -124,7 +124,7 @@ href="/src/runtime/runtime-gdb.py">src/runtime/runtime-gdb.py</a> in
 the Go source distribution. It depends on some special magic types
 (<code>hash&lt;T,U&gt;</code>) and variables (<code>runtime.m</code> and
 <code>runtime.g</code>) that the linker
-(<a href="/src/cmd/ld/dwarf.c">src/cmd/ld/dwarf.c</a>) ensures are described in
+(<a href="/src/cmd/link/internal/ld/dwarf.go">src/cmd/link/internal/ld/dwarf.go</a>) ensures are described in
 the DWARF code.
 </p>