Point "deref coercions" links to new book

Point "deref coercions" links to new book

Currently the link on doc.rust-lang.org is semi-broken; it links to a
page that links to the exact page in the first edition in the book, or
to the index of the second edition of the book. If the second editions
is the recommended one now, we should point the links at that one. It
seems that in the mean time, the links have been updated to point
directly to the first edition of the book, but that hasn't made it onto
the stable channel yet. By the time this commit makes it onto the stable
channel, the second edition of the book should be complete enough. At
least the part about deref coercions is.