docs: comment on explicit main branch match

src/serviceProviders/sourcehut.js

@@ -56,6 +56,7 @@ export function processURI (uri) {
         fetcher: E.Fetcher.HTTP,
         accessRestriction: E.ProofAccessRestriction.NONE,
         data: {
+          // explicitly matching for `main` branch
           url: `https://git.sr.ht/~${match[1]}/${match[2]}/blob/main/proof.md`,
           format: E.ProofFormat.TEXT
         }