Fixed a incorrect image URL for readme
Fixed a incorrect image URL for readme

wuxu/NMRangeSlider,namanhams/NMRangeSlider,mvelikov/NMRangeSlider,muZZkat/NMRangeSlider,Sega-Zero/NMRangeSlider,jfradj/NMRangeSlider,ivasic/NMRangeSlider,jiamaozheng/NMRangeSlider,neonaldo/NMRangeSlider