Fix failed attempt to format 'k' as subscript

chapters/stream.tex

@@ -94,7 +94,7 @@ \section{Stream Operator inStream and Connection Equations}\label{stream-operato
 
 For the following definition it is assumed that $N$ inside connectors
 \lstinline!$m_{j}$.c! ($j = 1, 2, \ldots, N$) and $M$ outside connectors
-\lstinline!$c{k}$! ($k = 1, 2, \ldots, M$) belonging to the same connection set
+\lstinline!$c_{k}$! ($k = 1, 2, \ldots, M$) belonging to the same connection set
 (see definition in \cref{inside-and-outside-connectors}) are connected
 together and a stream variable \lstinline!h_outflow! is associated with a flow
 variable \lstinline!m_flow! in connector \lstinline!c!.