Do mysqldump manpage:
o --password[=password], -p[password]
The password to use when connecting to the server. If you use the short option form (-p), you cannot have a space between the option and the password. If you omit the password value following the
--password or -p option on the command line, mysqldump prompts for one.
Specifying a password on the command line should be considered insecure. You can use an option file to avoid giving the password on the command line.
Apenas omita o valor da senha em -p
flag e o mysqldump solicitará a senha.