fix: update workflow to use pull_request_target (#25)

As noted here: https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/

With proposed doc PR in #24

.github/workflows/pull-request.yml

@@ -1,6 +1,6 @@
 name: 'Pull request'
 on:
-  pull_request:
+  pull_request_target:
     types:
       - opened
       - edited