blockhash: update meta.homepage

pkgs/tools/graphics/blockhash/default.nix

···
   strictDeps = true;
 
   meta = with lib; {
-    homepage = "https://blockhash.io/";
+    homepage = "https://github.com/commonsmachinery/blockhash";
     description = ''
       This is a perceptual image hash calculation tool based on algorithm
       descibed in Block Mean Value Based Image Perceptual Hashing by Bian Yang,