chore: fix changelog

Author: nebolsin

CHANGELOG.md

@@ -5,8 +5,6 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased](https://github.com/astroband/ruby-xdr/compare/v3.0.2...master)
-
 ## [3.0.2](https://github.com/astroband/ruby-xdr/compare/v3.0.1...v3.0.2) - 2020-10-20
 ### Changed
 - Relax lower bound for ActiveSupport dependency to support usage in Rails 4.2 ([#12](https://github.com/astroband/ruby-xdr/pull/12))