doc: specify root_volume size_in_gb to have a working example (#621)

Co-authored-by: Rémy Léone <[email protected]>

Author: vincentfretin

docs/index.md

@@ -87,6 +87,11 @@ resource "scaleway_instance_server" "web" {
 
   additional_volume_ids = [ scaleway_instance_volume.data.id ]
 
+  root_volume {
+    # The local storage of a DEV1-L instance is 80 GB, subtract 30 GB from the additional l_ssd volume, then the root volume needs to be 50 GB.
+    size_in_gb = 50
+  }
+
   security_group_id = scaleway_instance_security_group.www.id
 }
 ```