fix broken link from "About dataset" on heatmap app
fix broken link from "About dataset" on heatmap app

ucscXena/ucsc-xena-client,ucscXena/ucsc-xena-client,ucscXena/ucsc-xena-client,ucscXena/ucsc-xena-client,ucscXena/ucsc-xena-client,acthp/ucsc-xena-client,acthp/ucsc-xena-client,acthp/ucsc-xena-client