Edit user identifier examples: to @alice

From `@<username>`, `@your_username`, `@example`, etc.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

roles/custom/matrix-media-repo/templates/media-repo/media-repo.yaml.j2

@@ -172,7 +172,7 @@ admins:
   | to_nice_yaml(indent=2, width=999999, sort_keys=false) | indent(width=2, first=true) }}
 {% else %}
 #admins:
-#  - "@your_username:example.org"
+#  - "@alice:example.org"
 {% endif %}
 
 # Shared secret auth is useful for applications building on top of the media repository, such