README: use enttities for "^" to avoid replacing it by <sup>
README: use enttities for "^" to avoid replacing it by <sup>

onddo/mysql_tuning-cookbook,zuazo/mysql_tuning-cookbook,onddo/mysql_tuning-cookbook,onddo/mysql_tuning-cookbook,zuazo/mysql_tuning-cookbook,zuazo/mysql_tuning-cookbook