Update unclear wording in webpack config docs (facebook#3160)

Author: Kristie Howard

Diff for: packages/react-scripts/config/webpack.config.dev.js

@@ -199,7 +199,7 @@ module.exports = {
           // "file" loader makes sure those assets get served by WebpackDevServer.
           // When you `import` an asset, you get its (virtual) filename.
           // In production, they would get copied to the `build` folder.
-          // This loader don't uses a "test" so it will catch all modules
+          // This loader doesn't use a "test" so it will catch all modules
           // that fall through the other loaders.
           {
             // Exclude `js` files to keep "css" loader working as it injects

Diff for: packages/react-scripts/config/webpack.config.prod.js

@@ -218,7 +218,7 @@ module.exports = {
           },
           // "file" loader makes sure assets end up in the `build` folder.
           // When you `import` an asset, you get its filename.
-          // This loader don't uses a "test" so it will catch all modules
+          // This loader doesn't use a "test" so it will catch all modules
           // that fall through the other loaders.
           {
             loader: require.resolve('file-loader'),