test,doc: fix lint error in test fixtures

Add first-level header to .md file in fixture directory. The file in
question is documentation for the fixture and is not part of any tests.
This is in prepration for markdown linting of the `test` directory.

PR-URL: #22221
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>

test/fixtures/0-dns/README.md

@@ -1,3 +1,5 @@
+# 0-dns
+
 ## Purpose
 The test cert file for use `test/parallel/test-tls-0-dns-altname.js`
 can be created by using `asn1.js` and `asn1.js-rfc5280`,