Renames master to main in fixtures/flight webpack config file (#21998)

Author: junaidbinfarooq

fixtures/flight/config/webpack.config.js

@@ -608,7 +608,7 @@ module.exports = function(webpackEnv) {
       // This is necessary to emit hot updates (CSS and Fast Refresh):
       isEnvDevelopment && new webpack.HotModuleReplacementPlugin(),
       // Experimental hot reloading for React .
-      // https://github.com/facebook/react/tree/master/packages/react-refresh
+      // https://github.com/facebook/react/tree/main/packages/react-refresh
       isEnvDevelopment &&
         shouldUseReactRefresh &&
         new ReactRefreshWebpackPlugin({