Change bot username to bot.matrix-registration-bot following convention

roles/matrix-bot-matrix-registration-bot/defaults/main.yml

@@ -25,7 +25,7 @@ matrix_bot_matrix_registration_bot_systemd_wanted_services_list: []
 
 # The bot's username. This user needs to be created manually beforehand.
 # Also see `matrix_bot_matrix_registration_bot_user_password`.
-matrix_bot_matrix_registration_bot_matrix_user_id_localpart: "matrix-registration-bot"
+matrix_bot_matrix_registration_bot_matrix_user_id_localpart: "bot.matrix-registration-bot"
 
 matrix_bot_matrix_registration_bot_matrix_user_id: '@{{ matrix_bot_matrix_registration_bot_matrix_user_id_localpart }}:{{ matrix_domain }}'