readme typo: convert string to file

It's a simple change but it caused me a little time trying to find why had a cast error when there was no line number indicated in the error.
```
:requireJs

FAILURE: Build failed with an exception.

* What went wrong:
org.codehaus.groovy.runtime.GStringImpl cannot be cast to java.io.File

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 3.662 secs
```