Small Documentation problem in ofxXmlSettings

[adminfriso]

on the code block of "A more complex example involving saving an array."

positions.addTag("positions");

will result in indefinitely adding a new positions tag
adding a tag exists around it will make sure this will not happen.
should be a small fix for someone who already has this repo setup for editing

if (!positions.tagExists("positions"))
{
positions.addTag("positions");
}