README edit: remove ambiguity in an important 'Finer Points' tip

Hi there, I'm new to this library.  :)  I was checking the readme for best practices and came across this line.  I was a little confused at first when I read it.  I decided to tweak the readme to avoid any confusion.  It's only one word.  Totally not a big deal at all, but figured I'd contribute.  What do you guys think?  

Looking forward to digging more into overscroll!!  :)