Update README.md with protobuf version matching actual dependencies (bazel-contrib#1012)

Version taken from scala/private/macros/scala_repositories.bzl.
Previous version from docs was conflicting with actual version of zlib.

Author: gergelyfabian

README.md

@@ -55,8 +55,8 @@ scala_register_toolchains()
 load("@io_bazel_rules_scala//scala:scala.bzl", "scala_repositories")
 scala_repositories()
 
-protobuf_version="09745575a923640154bcf307fba8aedff47f240a"
-protobuf_version_sha256="416212e14481cff8fd4849b1c1c1200a7f34808a54377e22d7447efdf54ad758"
+protobuf_version="3.11.3"
+protobuf_version_sha256="cf754718b0aa945b00550ed7962ddc167167bd922b842199eeb6505e6f344852"
 
 http_archive(
     name = "com_google_protobuf",