Fix typos (#5599)

packages/next/README.md

@@ -1349,7 +1349,7 @@ module.exports = {
 }
 ```
 
-The second argument to `webpack` is an object containing properties useful when customing the WebPack configuration:
+The second argument to `webpack` is an object containing properties useful when customizing its configuration:
 
 - `buildId` - `String` the build id used as a unique identifier between builds
 - `dev` - `Boolean` shows if the compilation is done in development mode