Fix links and extra redundant header in README.md

Author: nerrad

README.md

@@ -13,8 +13,7 @@ Use this plugin if you want access to the bleeding edge of available blocks for
 - [Documentation](#documentation)
   - [Code Documentation](#code-documentation)
   - [Contributing](#contributing)
-- [Contributing](#contributing-1)
-- [Installing the plugin version](#installing-the-stable-version)
+- [Installing the plugin version](#installing-the-plugin-version)
 - [Installing the development version](#installing-the-development-version)
 - [Getting started with block development](#getting-started-with-block-development)
 - [Vision for the Feature](#vision-for-the-feature)
@@ -35,8 +34,6 @@ If you want to see what we're working on for future versions, or want to help ou
 
 - [Publishing a release](docs/releases/readme.md)
 
-## Contributing
-
 ## Installing the plugin version
 
 We release a new version of WooCommerce Blocks onto WordPress.org every few weeks, which can be used as an easier way to preview the features.