website: Update "Local Values" docs to use v0.12 syntax in example

There was some leftover v0.11-style interpolation syntax here.

We prefer to use a "naked" expression in situations like this where the result
isn't a string, because interpolations returning non-strings is a common source
of confusion for new users.

website/docs/configuration/locals.html.md

@@ -45,7 +45,7 @@ values from elsewhere in the module:
 ```hcl
 locals {
   # Ids for multiple sets of EC2 instances, merged together
-  instance_ids = "${concat(aws_instance.blue.*.id, aws_instance.green.*.id)}"
+  instance_ids = concat(aws_instance.blue.*.id, aws_instance.green.*.id)
 }
 
 locals {