CONTRIBUTING.md: Fixes datastore create command

`create-index` has been replaced with `indexes create`.

Change-Id: Ibe92586027fc75dc14551e840e49bf569142a605
Reviewed-on: https://code-review.googlesource.com/c/38150
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index eef41ed..b7e5e68 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -121,7 +121,7 @@
 $ gcloud auth login
 
 # Create the indexes used in the datastore integration tests.
-$ gcloud datastore create-indexes datastore/testdata/index.yaml
+$ gcloud datastore indexes create datastore/testdata/index.yaml
 
 # Creates a Google Cloud storage bucket with the same name as your test project,
 # and with the Stackdriver Logging service account as owner, for the sink