Don't give impression of undefined behavior being error

The word "invalid" sounds too much like something which is definitely an error, which isn't the case here.

chapters/operatorsandexpressions.tex

@@ -179,8 +179,8 @@ \section{Evaluation Order}\label{evaluation-order}
   Boolean b;
   Integer I;
 equation
-  b = (I >= 1 and I <= n) and v[I];                // Invalid
-  b = if (I >= 1 and I <= n) then v[I] else false; // Correct
+  b = (I >= 1 and I <= n) and v[I];                // May result in error
+  b = if (I >= 1 and I <= n) then v[I] else false; // Safe
 \end{lstlisting}
 
 To guard square against square root of negative number use \lstinline!noEvent!: