Add bot command prefix default to main.yml

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

@@ -43,6 +43,9 @@ matrix_bot_matrix_registration_bot_matrix_user_id: '@{{ matrix_bot_matrix_regist
 # The bot's password (can also be used to login via a client like Element Web)
 matrix_bot_matrix_registration_bot_bot_password: ''
 
+# Optional bot command prefix
+matrix_bot_matrix_registration_bot_bot_prefix: ""
+
 # Homeserver base URL
 matrix_bot_matrix_registration_bot_api_base_url: "{{ matrix_homeserver_url }}"