Update preview.yaml

Author: jcmartinezdev

.github/workflows/preview.yaml

@@ -3,6 +3,8 @@ env:
   VERCEL_ORG_ID: ${{ secrets.VERCEL_ORG_ID }}
   VERCEL_PROJECT_ID: ${{ secrets.VERCEL_PROJECT_ID }}
 on:
+  pull_request:
+    branches: [master]
   push:
     branches-ignore:
       - master