tip/doc: update doc to match latest gcloud syntax
tip/doc: update doc to match latest gcloud syntax

gcloud command no longer requires preview for deployment and is showing
warning when command is executed:

```
WARNING: The `gcloud preview app` command group is deprecated; please
use the `gcloud app` commands instead.
```

This change is removing "preview" text from README to make deploy
command up to date with gcloud.

Change-Id: I64d390ab4891f82fe608c64be20fae01a80d00fc
Reviewed-on: https://go-review.googlesource.com/25430
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools,golang/tools,golang/tools,golang/tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,syreclabs/go-tools