Fix matrix_homeserver_admin_contacts example

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1931

This does 2 things:

- it fixes the syntax for `matrix_id`. Starting strings with `@` is
  invalid YAML, so such strings need to be wrapped in single or double
  quotes
- it makes use of the `matrix_domain` variable instead of hardcoding the
  domain name. This should be more and mistake-proof (typos or people
  mistaking their domain - matrix. vs base domain)

docs/configuring-well-known.md

@@ -48,11 +48,11 @@ Example snippet for `vars.yml`:
 ```
 # Homeserver admin contacts as per MSC 1929 https://github.com/matrix-org/matrix-spec-proposals/pull/1929
 matrix_homeserver_admin_contacts:
-  - matrix_id: @admin1:domain.tld
+  - matrix_id: "@admin1:{{ matrix_domain }}"
     email_address: admin@domain.tld
     role: admin
-  - matrix_id: @admin2:domain.tld
+  - matrix_id: "@admin2:{{ matrix_domain }}"
-    email_address: admin@domain.tld
+    email_address: admin2@domain.tld
     role: admin
   - email_address: security@domain.tld
     role: security