present: update link to sam documentation
present: update link to sam documentation

The link to the sam documentation explaining the use
of the address syntax used when loading code files
was dead. This updates the location of the
documentation to the new plan9 site.

Fixes golang/go#23247

Change-Id: If27702a17700ac859650e1e3c070cb43345f32b0
Reviewed-on: https://go-review.googlesource.com/85420
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools