Fix a typo

book.tex

@@ -304,7 +304,7 @@ integer is an expression, $\Exp$, in the language:
 \end{equation}
 Each rule has a left-hand-side and a right-hand-side. The way to read
 a rule is that if you have all the program parts on the
-right-hand-side, then you can create and AST node and categorize it
+right-hand-side, then you can create an AST node and categorize it
 according to the left-hand-side. (We do not define $\Int$ because the
 reader already knows what an integer is.) We make the simplifying
 design decision that all of the languages in this book only handle