Shorten long heading to make "String Concatenation" more visible

As there is only element-wise addition and subtraction, shortening the heading doesn't introduce any conflicting overlap between headings.

chapters/arrays.tex

@@ -1079,7 +1079,7 @@ \subsection{Equality and Assignment}\label{equality-and-assignment}
 \end{center}
 \end{table}
 
-\subsection{Element-wise Addition, Subtraction, and String Concatenation}\label{array-element-wise-addition-subtraction-and-string-concatenation}\label{element-wise-addition-subtraction-and-string-concatenation}
+\subsection{Addition, Subtraction, and String Concatenation}\label{array-element-wise-addition-subtraction-and-string-concatenation}\label{addition-subtraction-and-string-concatenation}
 
 Addition \lstinline!a + b! and subtraction \lstinline!a - b! of numeric scalars, vectors, matrices,
 and arrays is defined element-wise and require \lstinline!size(a) = size(b)! and a