Add print function calls in 6.1 and 6.8

Author: lbordoni

book/book.tex

@@ -4578,7 +4578,7 @@ \section{Return values}
 \index{special value!None}
 
 \begin{verbatim}
->>> absolute_value(0)
+>>> print(absolute_value(0))
 None
 \end{verbatim}
 %
@@ -5122,10 +5122,10 @@ \section{Checking types}
 went wrong:
 
 \begin{verbatim}
->>> factorial('fred')
+>>> print(factorial('fred'))
 Factorial is only defined for integers.
 None
->>> factorial(-2)
+>>> print(factorial(-2))
 Factorial is not defined for negative integers.
 None
 \end{verbatim}