typo

book.tex

@@ -9765,7 +9765,7 @@ cases for \code{Apply} and \code{FunRef} and the function for
 predicate context should be updated for \code{Apply} but not
 \code{FunRef}.  (A \code{FunRef} can't be a Boolean.) \ocaml{The predicate
 context treatment of \code{Apply} will need to be handled similarly
-to \code{SetField} introduced in \LangTuple{}. Neither of the new forms
+to \code{GetField} introduced in \LangTuple{}. Neither of the new forms
 should be added to the function for effectful contexts (a \code{FunRef}
 cannot have type Void, and our \LangFun{} type checker prohibits
 functions from having return type Void too).}