fix(ci): error in message-relayer canary build (#3478)

Fixes a small error in the message-relayer canary action that would
cause it to fail to build.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/gold-singers-thank.md

@@ -0,0 +1,5 @@
+---
+'@eth-optimism/message-relayer': patch
+---
+
+Small patch release meant to test a fix for a bug in our canary release process for this package.

.github/workflows/publish-canary.yml

@@ -200,7 +200,7 @@ jobs:
         with:
           context: .
           file: ./ops/docker/Dockerfile.packages
-          target: relayer
+          target: message-relayer
           push: true
           tags: ethereumoptimism/message-relayer:${{ needs.canary-publish.outputs.canary-docker-tag }}