Fix typo

src/custom.tex

@@ -251,7 +251,7 @@ \subsection{New Environments}
     command.}\label{lst:kingenv}
 \end{listing}
 
-Note that when environment argument are read, they are read \emph{after} the
+Note that when environment arguments are read, they are read \emph{after} the
 \csi{begin}[name: m] command. This may be especially counterintuitive when we
 consider the \cargv{s} specification. \autoref{lst:kingsenv} illustrates this.
 \begin{listing}