methods/data: add GTEx v8 URL

content/08.05.methods.data.md

@@ -1,5 +1,5 @@
 ### Gene expression data and preprocessing {#sec:data_gtex}
 
-We downloaded GTEx v8 data for all tissues, normalized using TPM (transcripts per million), and focused our primary analysis on whole blood, which has a good sample size (755).
-We selected the top 5,000 genes from whole blood with the largest variance after standardizing with $log(x + 1)$ to avoid a bias towards highly-expressed genes.
-We then computed Pearson, Spearman, MIC and CCC on these 5,000 genes across all 755 samples on the TPM-normalized data, generating a pairwise similarity matrix of size 5,000 x 5,000.
+We downloaded gene expression data from GTEx v8 ([https://gtexportal.org/](https://gtexportal.org/)) for all tissues, normalized using TPM (transcripts per million), and focused our primary analysis on whole blood, which has a good sample size (755).
+We selected the top 5,000 genes from whole blood with the largest variance after standardizing with $log(x + 1)$ to avoid a bias toward highly expressed genes.
+We then computed Pearson, Spearman, MIC and CCC on these 5,000 genes across all 755 samples, generating a pairwise similarity matrix of size 5,000 x 5,000.