fix(talk,coturn): correct vars file header (was 'httpbin')

Both new roles had 'vars file for httpbin' as the header comment in
vars/main.yml — copy-paste artefact from the httpbin role template.
Files are otherwise empty. Reviewer flagged both inline (PR review
comments 229 and 230).

roles/coturn/vars/main.yml

@@ -1,3 +1,3 @@
 #SPDX-License-Identifier: MIT-0
 ---
-# vars file for httpbin
+# vars file for coturn

roles/talk/vars/main.yml

@@ -1,3 +1,3 @@
 #SPDX-License-Identifier: MIT-0
 ---
-# vars file for httpbin
+# vars file for talk