Fix Serial2 inclusion code

After reading about Serial2 on the docs I wanted to try it out, but `#include "Serial2.h"` wasn't working. After some digging, I found that the proper way to include libraries is `#include "Serial2/Serial2.h"` . The documentation doesn't yet reflect this, and thus this commit :)
Merge pull request #231 from Pirate43/patch-1

Fix Serial2 inclusion code