doc: rewrite reference to plan9.bell-labs.com to 9p.io

Change-Id: I75619feced842b8ca509ee08e01b63258c5e87ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/190757
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: wgrr

doc/go1.3.html

@@ -373,7 +373,7 @@ <h3 id="new_packages">New packages</h3>
 
 <p>
 A new package <a href="/pkg/debug/plan9obj/"><code>debug/plan9obj</code></a> was added to the standard library.
-It implements access to Plan 9 <a href="http://plan9.bell-labs.com/magic/man2html/6/a.out">a.out</a> object files.
+It implements access to Plan 9 <a href="https://9p.io/magic/man2html/6/a.out">a.out</a> object files.
 </p>
 
 <h3 id="major_library_changes">Major changes to the library</h3>