Grant plan action the write permissions for diffs

Without the write permission the diffs were invalid.

Source:
https://registry.terraform.io/providers/integrations/github/latest/docs/resources/repository

Author: tim-schilling

.github/workflows/plan.yml

@@ -13,7 +13,7 @@ jobs:
 
     permissions:
       pull-requests: write
-      contents: read
+      contents: write
 
     steps:
       - name: Checkout code