docs(httpauth-firebase): fix rbac tier reference from 1 to 0

rbac is a Tier 0 module (no micro-lib dependencies). The dependency line
incorrectly cited it as Tier 1. The module's own tier (4) is unchanged —
it remains the auth layer above the transport infrastructure.

go.mod

@@ -0,0 +1,39 @@
+module code.nochebuena.dev/go/httpauth-firebase
+
+go 1.25
+
+require (
+	code.nochebuena.dev/go/rbac v0.9.0
+	firebase.google.com/go/v4 v4.15.0
+)
+
+require (
+	cloud.google.com/go/compute v1.24.0 // indirect
+	cloud.google.com/go/compute/metadata v0.2.3 // indirect
+	github.com/felixge/httpsnoop v1.0.4 // indirect
+	github.com/go-logr/logr v1.4.1 // indirect
+	github.com/go-logr/stdr v1.2.2 // indirect
+	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
+	github.com/golang/protobuf v1.5.4 // indirect
+	github.com/google/s2a-go v0.1.7 // indirect
+	github.com/googleapis/enterprise-certificate-proxy v0.3.2 // indirect
+	go.opencensus.io v0.24.0 // indirect
+	go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc v0.49.0 // indirect
+	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.49.0 // indirect
+	go.opentelemetry.io/otel v1.24.0 // indirect
+	go.opentelemetry.io/otel/metric v1.24.0 // indirect
+	go.opentelemetry.io/otel/trace v1.24.0 // indirect
+	golang.org/x/crypto v0.21.0 // indirect
+	golang.org/x/net v0.23.0 // indirect
+	golang.org/x/oauth2 v0.18.0 // indirect
+	golang.org/x/sync v0.6.0 // indirect
+	golang.org/x/sys v0.18.0 // indirect
+	golang.org/x/text v0.14.0 // indirect
+	golang.org/x/time v0.5.0 // indirect
+	google.golang.org/api v0.170.0 // indirect
+	google.golang.org/appengine v1.6.8 // indirect
+	google.golang.org/appengine/v2 v2.0.2 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20240311132316-a219d84964c2 // indirect
+	google.golang.org/grpc v1.62.1 // indirect
+	google.golang.org/protobuf v1.33.0 // indirect
+)