Add missing $ in ref, and add fallback to main (#14174)

### WHY are these changes introduced?
Fix to workflow file


### WHAT is this pull request doing?
Fixes typo and adds fallback to main for new work flow.

Author: mpshopify

.github/workflows/deploy-polaris.shopify.com.yml

@@ -27,5 +27,5 @@ jobs:
               owner: "shopify",
               repo: "polaris-site-prod-kit",
               workflow_id: "build-and-deploy.yml",
-              ref: "{{ inputs.branch }}",
+              ref: "${{ inputs.branch || 'main' }}",
             });