preview.env.example: flotilla-core overlay set (not the retired per-app shim)

The example comment referenced 'ohm-rfc-app-flotilla overlay set' — the retired
per-app shim. flotilla-core is the operator CLI for all deployments.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

deploy/preview/preview.env.example

@@ -5,7 +5,7 @@
 #
 #     while IFS='=' read -r k v; do
 #       [ -n "$k" ] && case "$k" in \#*) ;; *) \
-#         ohm-rfc-app-flotilla overlay set ohm-rfc-app "$k=$v" --preview ;; esac
+#         flotilla-core overlay set <deployment> "$k=$v" --preview ;; esac
 #     done < deploy/preview/preview.env.example
 #
 # CRITICAL (§15 / §3 invariant 1): a preview resolves ZERO real secret bytes.