ci: fix the permissions of the assign action

Fixes: rook#14518

A recent change seems to have broken the permissions of the auto-assign
action.

This tries to fix this by making permissions more specific.

Signed-off-by: Michael Adam <obnox@samba.org>

.github/workflows/auto-assign.yaml

@@ -10,7 +10,7 @@ jobs:
   assign:
     permissions:
       # write permissions are needed to assign the issue.
-      contents: write
+    issues: write
     name: Run self assign job
     runs-on: ubuntu-latest
     steps: