Update readme to point to the latest version

Fixes #123

README.md

diff --git a/README.md b/README.md
index e34dc33..529b1f8 100644
--- a/README.md
+++ b/README.md
@@ -19,11 +19,10 @@
 
 http_archive(
     name = "rules_proto",
-    sha256 = "66bfdf8782796239d3875d37e7de19b1d94301e8972b3cbd2446b332429b4df1",
-    strip_prefix = "rules_proto-4.0.0",
+    sha256 = "e017528fd1c91c5a33f15493e3a398181a9e821a804eb7ff5acdd1d2d6c2b18d",
+    strip_prefix = "rules_proto-4.0.0-3.20.0",
     urls = [
-        "https://mirror.bazel.build/github.com/bazelbuild/rules_proto/archive/refs/tags/4.0.0.tar.gz",
-        "https://github.com/bazelbuild/rules_proto/archive/refs/tags/4.0.0.tar.gz",
+        "https://github.com/bazelbuild/rules_proto/archive/refs/tags/4.0.0-3.20.0.tar.gz",
     ],
 )
 load("@rules_proto//proto:repositories.bzl", "rules_proto_dependencies", "rules_proto_toolchains")