update greetings.yml with pull_request_target (#677)

Should resolve "Resource not accessible by integration" error message. https://github.com/actions/first-interaction/issues/10#issuecomment-670968624

update greetings.yml with pull_request_target (#677)
0e207551 · Glenn Jocher · GitHub · e742dcf4 · 0e207551
--- a/.github/workflows/greetings.yml
+++ b/.github/workflows/greetings.yml
 name: Greetings
-on: [pull_request, issues]
+on: [pull_request_target, issues]
 jobs:
  greeting: