Name the production Vault after the exch product

The internal name of the product is exch, like its production server.
Paths, the Compose project, the Docker network, raft node ids and the
transit seal key and policy use it before the first rollout, so nothing
has to be renamed once data exists.

infra/prod/vault/gamma.yml

@@ -1,6 +1,6 @@
-# SXCHNGE production Vault on infra-gamma: node 3 of three (#74). It keeps a
+# Production Vault on infra-gamma: node 3 of three (#74). It keeps a
 # copy of the data off exch; nodes 1 and 2 run on exch (exch.yml).
-name: sxchnge-vault
+name: exch-vault
 
 services:
   vault-3: