Fix plugin slug name in workflow configuration

Author: sc0ttkclark

.github/workflows/plugin-check-stats.yml

@@ -14,7 +14,7 @@ jobs:
       fail-fast: false
       matrix:
         plugin_slug:
-          - advanced-custom-field
+          - advanced-custom-fields
           - custom-post-type-ui
           - meta-box
           - cmb2