issue template: fill 'about' field

according to [GitHub documentation] some fields are required.
`about` is not listed, but it probably is required.

[GitHub documentation]: https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms

.github/ISSUE_TEMPLATE/missing_documentation.md

@@ -1,8 +1,8 @@
 ---
 name: Missing or incorrect documentation
-about:
+about: Help us improve the reference manual
 title: ''
-labels: 'documentation'
+labels: documentation
 assignees: ''
 
 ---