github: fix config mistake preventing repo lockdown commenting

The previous commit updated the repo lockdown config to the new
format:

  commit 9b89cdb2a5
  Author: Alex Bennée <alex.bennee@linaro.org>
  Date:   Mon Oct 4 16:43:08 2021 +0100

    .github: move repo lockdown to the v2 configuration

Unfortunately the config key names used were wrong, resulting in
the repo lockdown throwing warnings:

  Unexpected input(s) 'pull-comment', 'lock-pull', 'close-pull',
  valid inputs are ['github-token', 'exclude-issue-created-before',
  'exclude-issue-labels', 'issue-labels', 'issue-comment',
  'skip-closed-issue-comment', 'close-issue', 'lock-issue',
  'issue-lock-reason', 'exclude-pr-created-before', 'exclude-pr-labels',
  'pr-labels', 'pr-comment', 'skip-closed-pr-comment', 'close-pr',
  'lock-pr', 'pr-lock-reason', 'process-only', 'log-output']

It still locked down the pull requests, due to its default config,
but didn't leave the friendly message explaining why.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

.github/workflows/lockdown.yml

@@ -15,7 +15,7 @@ jobs:
     steps:
       - uses: dessant/repo-lockdown@v2
         with:
-          pull-comment: |
+          pr-comment: |
             Thank you for your interest in the QEMU project.
 
             This repository is a read-only mirror of the project's repostories hosted
@@ -26,5 +26,5 @@ jobs:
             functionality). However, we get a lot of patches, and so we have some
             guidelines about contributing on the project website:
             https://www.qemu.org/contribute/
-          lock-pull: true
-          close-pull: true
+          lock-pr: true
+          close-pr: true