update readme example

Input File (`example_chart.plot`):

    width: 1200px;

    height: 480px;

    plot {

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

      x: month;

      y: temperature;

      group: city;

      axis-y-min: -10;

      axis-y-max: 32;

      layer {

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

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

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

        type: lines;

        stroke: 2pt;

      }

      legend {

        position: top left inside;

      }

    }