fix MATLAB grouped box plot example

[BRONSOLO]

need to add { } brackets to x values within traces given: http://stackoverflow.com/questions/939544/matlab-bug-or-really-weird-behavior-with-structs-and-empty-cell-arrays

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x = {'day 1' 'day 1' 'day 1' 'day 1' 'day 1' 'day 1' ...
     'day 2' 'day 2' 'day 2' 'day 2' 'day 2' 'day 2'}

trace1 = struct(...
  'y', [0.2, 0.2, 0.6, 1.0, 0.5, 0.4, 0.2, 0.7, 0.9, 0.1, 0.5, 0.3], ...
  'x', {x}, ...
  'name', 'kale', ...
  'marker', struct('color', '#3D9970'), ...
  'type', 'box');
trace2 = struct(...
  'y', [0.6, 0.7, 0.3, 0.6, 0.0, 0.5, 0.7, 0.9, 0.5, 0.8, 0.7, 0.2], ...
  'x', {x}, ...
  'name', 'radishes', ...
  'marker', struct('color', '#FF4136'), ...
  'type', 'box');
trace3 = struct(...
  'y', [0.1, 0.3, 0.1, 0.9, 0.6, 0.6, 0.9, 1.0, 0.3, 0.6, 0.8, 0.5], ...
  'x', {x}, ...
  'name', 'carrots', ...
  'marker', struct('color', '#FF851B'), ...
  'type', 'box');
data = {trace1, trace2, trace3};
layout = struct(...
    'yaxis', struct(...
      'title', 'normalized moisture', ...
      'zeroline', false), ...
    'boxmode', 'group');
response = plotly(data, struct('layout', layout, 'filename', 'box-grouped', 'fileopt', 'overwrite'));
plot_url = response.url