typo: fix contraction of "let us"

[dijkstracula]

Hi Ranjit and others,

Thanks again for providing this tutorial; working through it has been elucidating.

This is a total nit so forgive me, but, I noticed a repeated typo as I was working through subsequent chapters: the contraction of "let us" is "let's", not "lets" (the latter being the third person singular conjugation of "to let"). Skimming the literate haskell docs, I don't see anything about not being able to use single quotes in certain contexts so I just made the changes as I came across them.

Cheers, Nathan

[ranjitjhala]

Thanks so much, I don't know why I missed your PR apologies for the delay!

[dijkstracula]

Not a problem! Cheers!