simplify readme example

Input File (`example_chart.plot`):

    plot {

      data: csv('testdata/city_temperatures.csv');

      x: var(month);

      y: var(temperature);

      group: var(city);

      axis-y-min: -10;

      axis-y-max: 32;

      layer {

        type: lines;

      }

      layer {

        type: points;

        size: 3.3pt;

        x: csv('data.csv', month);

        y: csv('data.csv', temperature);

        group: csv('data.csv', city);

      }

    }