pubsys: minor comment changes in Infra.toml

* Corrects the env variable to override for changing the `repo` pubsys
reads from. Should be `PUBLISH_REPO` and not `REPO`.
* Adds a similar comment for changing the `signing-key` used by pubsys.

tools/pubsys/Infra.toml.example

@@ -9,14 +9,16 @@ root_role_path = "/home/user/root.json"
 # as part of the initial setup of your TUF roles.  pubsys assumes a single
 # publication key that lives in the snapshot, targets, and timestamp roles.
 # Here you specify where that key lives so we can sign the created repo.
+# You can specify multiple keys, if you like, and select one by name:
+#     cargo make repo -e PUBLISH_KEY=mysigningkey
 # (Don't keep your keys in the repo!)
 # You can keep your private key in a file or an SSM parameter; pick one:
 # (Need inline table syntax until this is fixed: https://github.com/alexcrichton/toml-rs/issues/225)
 signing_keys = { default = { file = { path = "/home/user/key.pem" } } }
 # signing_keys = { default = { ssm = { parameter = "/my/parameter" } } }
 
 # You can have any number of repos defined and build a specific one by running like this:
-#     cargo make repo -e REPO=myrepo
+#     cargo make repo -e PUBLISH_REPO=myrepo
 # If the URLs are uncommented, they will be pulled and used as a starting
 # point, and your images (and related files) will be added as a new update in
 # the created repo.  Otherwise, we build a new repo from scratch.