fix minor typos in window-functions.Rmd

This is just a fix of two minor typos in the Window Functions vignette:

1. The example shown (line 23) refers to hits, rather than home runs. Here is the gist to see the relevant setup for this example: https://gist.github.com/justmarkham/9490269

2. The "; TRUE" at the end of line 140 appears to serve no purpose.

Thanks!