my.cnf: typo fix (bind-address) + migrate key_buffer (deprecated) to key_buffer_size

Fix a typo in the config file where we can find bind_adress instead of bind-address (cf. https://dev.mysql.com/doc/refman/5.5/en/server-options.html#option_mysqld_bind-address [Option-File Format])

It also change the key_buffer which is deprecated into key_buffer_size (warning issued during installation on Ubuntu 12.04 machine).