_content/doc: update testdir import path in contribution guide

It's changed slightly to be inside 'cmd'.

For golang/go#56844.
For golang/go#60059.

Change-Id: I244f0ae627978a7b59d6a56d20aebc3ff81b3179
Reviewed-on: https://go-review.googlesource.com/c/website/+/493915
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
diff --git a/_content/doc/contribute.html b/_content/doc/contribute.html
index 1037dda..c97acd0 100644
--- a/_content/doc/contribute.html
+++ b/_content/doc/contribute.html
@@ -1167,7 +1167,7 @@
 by <code>all.bash</code> but you can also run it manually:
 
 <pre>
-$ $GOROOT/bin/go test internal/testdir
+$ $GOROOT/bin/go test cmd/internal/testdir
 </pre>
 </ul>