Fix F.RedeemablePromise code samples

I introduced an error in javadoc in #1999. I couldn't reproduce on my workstation and cloudbees didn't reproduced either. But it did produce on @yanns environment.
The error is the following one:
```
[error] /home/simon/projects/play/Play20/framework/src/play/src/main/java/play/libs/F.java:448: error: bad use of '>'
[error]      * someFutureInt.map(new Function<Int, Result>{
```


It look like was caused by wrong example code. While I have been unable to reproduce, the example code should be fixed. Here is the fix.