website: Fix confusing example for local-exec provisioner

The local-exec provisioner documentation includes an example which refers
to an attribute of the current resource using its full traversal path,
rather than using "self" as we typically expect.

Due to some coincidences in how Terraform builds the dependency graph,
referring to the resource in this way happens to work when the resource
has only a single instance (the graph builder just skips that
self-referential dependency edge), but it fails if the user later tries
to add "count" or "for_each" to the resource, because at that point all
of the instances become dependent on one another, which creates a
dependency cycle.

Using "self" to access the current instance attributes is the usual
approach, so I've updated the documentation to show that.

website/docs/provisioners/local-exec.html.markdown

@@ -29,7 +29,7 @@ resource "aws_instance" "web" {
   # ...
 
   provisioner "local-exec" {
-    command = "echo ${aws_instance.web.private_ip} >> private_ips.txt"
+    command = "echo ${self.private_ip} >> private_ips.txt"
   }
 }
 ```