Update legend.md (#8582)

Noticed from https://github.com/chartjs/Chart.js/issues/6185#issuecomment-748097030 that there is an option to get the legend in the chart, this was not documented. Only difference I noticed from kurkles screenshot is that the legend now shows in the middle instead of in the top corner.

https://jsfiddle.net/Leelenaleee/wszdv8q0/2/