fix sectioning

Author: zingale

intro/intro.tex

@@ -127,7 +127,7 @@ \subsection{Differentiation and Integration}
   notes, we'll be concerned only with the first case.
 \end{enumerate}
 
-\subsection{Differentiation}
+\subsubsection{Differentiation}
 \label{ch:intro:diff}
 Consider a collection of equally spaced points, labeled with an index
 $i$, with the physical spacing between them denoted $\Delta x$.  We
@@ -210,7 +210,7 @@ \subsection{Differentiation}
 Second- and higher-order derivatives can be constructed in the same fashion.
 
 
-\subsection{Integration}
+\subsubsection{Integration}
 
 In numerical analysis, any integration method that is composed as a
 weighted sum of the function evaluated at discrete points is called a