Update api.md
Update api.md

Fixed a typo mentioned in this issue https://github.com/igvteam/igv.js/issues/161
NabaviLab/CNV-Visualizer,NabaviLab/CNV-Visualizer,igvteam/igv.js,NabaviLab/CNV-Visualizer,NabaviLab/CNV-Visualizer,igvteam/igv.js,NabaviLab/CNV-Visualizer