formatting: add spaces at the end of 2 Racket-specific sentences

Spaces are missing at these location in the print/PDF version.

book.tex

@@ -560,7 +560,7 @@ called \emph{parsing}\index{subject}{parsing}\python{\ and is studied in
   chapter~\ref{ch:parsing}}.
 \racket{This book does not cover the theory and implementation of parsing.
   We refer the readers interested in parsing to the thorough treatment
-  of parsing by \citet{Aho:2006wb}.}%
+  of parsing by \citet{Aho:2006wb}. }%
 %
 \racket{A parser is provided in the support code for translating from
   concrete to abstract syntax.}%
@@ -995,7 +995,7 @@ combine several right-hand sides into a single rule.
 
 The concrete syntax for \LangInt{} is shown in
 figure~\ref{fig:r0-concrete-syntax} and the abstract syntax for
-\LangInt{} is shown in figure~\ref{fig:r0-syntax}.%
+\LangInt{} is shown in figure~\ref{fig:r0-syntax}. %
 %
 \racket{The \code{read-program} function provided in
   \code{utilities.rkt} of the support code reads a program from a file