harmonize plurality across template names

Author: wion

.github/ISSUE_TEMPLATE/discuss-existing-page.md

@@ -1,5 +1,5 @@
 ---
-name: Existing pages
+name: Existing page
 about: Use to discuss or update task objective for an existing doc page that does not already have a dedicated Issue (i.e. search Issues before using this template)
 title: 'page: '
 labels: ''