Point "PuresScript by Example" links to book.purescript.org

[kurtmilam]

By my count, there are are five links in the documentation that point to the book on leanpub.

I'd be happy to open a PR if there's interest.

Pages with links to leanpub (may not be exhaustive):

• README.md#project-scope
• guides/FFI-Tips.md#why-doesnt-my-eff-work-when-passed-to-a-normal-js-function
• guides/FFI.md#conclusion
• language/Type-Classes.md#multi-parameter-type-classes
• language/Type-Classes.md#functional-dependencies

[hdgarrood]

Yes please!

[thomashoneyman]

This can be closed now that #316 has been merged.