env/plan9-386: add missing command in README

The ./make.bash command to create the image was missing, making the
text hard to understand.

Also modify the gcloud compute command example to use plan9-386-v6
as the VM image name, so that the example below makes more sense:

> Then update x/build/dashboard/builders.go with the new image version
> (e.g. "plan9-386-v5" to "plan9-386-v6").

Change-Id: I8c00ce4f255cb2dbff090539b3aafc99ffba2c4b
Reviewed-on: https://go-review.googlesource.com/129995
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

env/plan9-386/README

@@ -21,12 +21,13 @@ Also, due to an ATA bug affecting QEMU 1.6 and 1.7, the
 Plan 9 CD can't be booted with these versions.
 
 To create the image:
+  $ ./make.bash
 
 Then:
   $ gsutil cp -a public-read plan9-386-gce.tar.gz gs://go-builder-data/plan9-386-gce.tar.gz
 
 Then:
-  $ gcloud compute --project symbolic-datum-552 images create plan9-386-v5 --source-uri gs://go-builder-data/plan9-386-gce.tar.gz
+  $ gcloud compute --project symbolic-datum-552 images create plan9-386-v6 --source-uri gs://go-builder-data/plan9-386-gce.tar.gz
 
 To test the image before deploying it to prod, use x/build/cmd/debugnewvm.