Remove duplicate underscore ("_") from alphabet in lib/generator.js

I noticed the alphabet being used by the generator was 65 characters long instead of 64, and that there was an extra underscore ("_") at the end of the string. This pull request removes the extra character, making the alphabet 64 characters long again.

It _shouldn't_ technically matter whether it's there or not (since the randomization is bound to a range from 1-64), but I figured "better safe than sorry."