python-Electrum: fix file sha256

It's unfortunate that this changed, but the tarball does appear to have
a valid signature.

pkgs/applications/misc/electrum/default.nix

···
 
   src = fetchurl {
     url = "https://download.electrum.org/Electrum-${version}.tar.gz";
-    sha256 = "0z5ksr1wlywl4bpvxjmmqnsk7jh1jfjdz9lsjkhf2j391jx0wz9q";
+    sha256 = "0y04m5b410y3s9vqvkbvmlvvx1nr0cyvrnl41yapz8hydw9vdkjx";
   };
 
   propagatedBuildInputs = with pythonPackages; [