better example

src/math.tex

@@ -160,7 +160,7 @@ \subsection{Math Mode}
 now be used to typeset \wi{superscript}s and \wi{subscript}s, respectively.
 \begin{example}
 \( a^2 + b^2 = c^2 \) \\
-\( A_x = G_{\text{foo}} \)
+\( A_x = G_{a\times c} \)
 \end{example}
 
 \section{Building Blocks of Mathematical Formulae}