Use valid instead of legal (babel#297) [skip ci]

Legal makes it sounds like it's a legal issue, yet it's more about the validity of the value.

README.md

@@ -27,7 +27,7 @@ module: {
     {
       test: /\.js$/,
       exclude: /(node_modules|bower_components)/,
-      loader: 'babel', // 'babel-loader' is also a legal name to reference
+      loader: 'babel', // 'babel-loader' is also a valid name to reference
       query: {
         presets: ['es2015']
       }