Change to only beta

Author: Pavithra K

content/guides/migrating.md

@@ -142,7 +142,7 @@ It's no longer necessary to specify it in configuration.
 
 [ExtractTextPlugin](https://github.com/webpack/extract-text-webpack-plugin) 1.0.0 does not work with webpack v2. You will need to install ExtractTextPlugin v2 explicitly.
 
-`npm install --save-dev extract-text-webpack-plugin@2.0.0-beta.4`
+`npm install --save-dev extract-text-webpack-plugin@beta`
 
  The configuration changes for this plugin are mainly syntactical.