Update highcharts-convert.js

Running the converter while setting the parameter globaloptions results in the following error message:

Highcharts.options.parsed
SyntaxError: Parse error

Highcharts.customCode.parsed
ReferenceError: Can't find variable: globalOptions

  phantomjs://webpage.evaluate():79
  phantomjs://webpage.evaluate():132
  phantomjs://webpage.evaluate():132
TypeError: 'null' is not an object (evaluating 'svg.imgUrls')

  /usr/local/phantomjs/lib/highcharts/highcharts-convert.js:228
  /usr/local/phantomjs/lib/highcharts/highcharts-convert.js:503
  :/modules/webpage.js:281

This is due to the wrong variable name globalOptions which should be globalOptionsArg