Make the sentence more clear (#1405)

Author: phuctvt

packages/custom-webpack/README.md

@@ -380,7 +380,7 @@ In this case, the behavior will be the same as when exporting a plain object —
 If `customWebpackConfig.path` file exports a function, the behaviour of the builder changes : no more automatic merge is applied, instead the function
 is called with the base Webpack configuration and must return the new configuration.
 
-The function is called with the base config the builder options and the target options as parameters.
+The function is called with the base config, the builder options, and the target options as parameters.
 `TargetOptions` follows `target` definition from [this](https://github.com/angular/angular-cli/blob/master/packages/angular_devkit/architect/src/input-schema.json) schema
 and can be used to manipulate your build based on the build target.