Fixing spelling/grammar mistakes

Hey Andreas,

    I met you shortly at the SF Bitcoin meeting today, where you told me that your book is on Github, so I decided to do a thorough review of it. Here are a bunch of grammatical/spelling fixes in the first two chapters that I'd like to suggest. I will read the rest tomorrow.

    I also have some general comments about some paragraphs, but those are better sent in an email later this week.

Cheers,
Minh T. Nguyen.