[Typo] Sect 4.5 "there's no separate notions"

[mars0i]

Please quote the complete incorrect sentence - this makes it easy to find in the source code of the book.

Section 4.5:

Because Lean is a pure functional language, there's no separate notions of reference vs value equality—pointers cannot be observed directly.

"notions" should be "notion" (or "there's" should be "there are").