Add suggested 'A' at start of sentence and go singular

Co-authored-by: Hans Olsson <HansOlsson@users.noreply.github.com>

chapters/statements.tex

@@ -180,7 +180,7 @@ \subsection{For-statement}\label{for-statement}
   { statement ";" }
 end for
 \end{lstlisting}
-\lstinline!for!-statements may optionally use several iterators (\lstinline!for-indices!), see \cref{nested-for-loops-and-reduction-expressions-with-multiple-iterators} for more information:
+A \lstinline!for!-statement may optionally use several iterators (\lstinline!for-indices!), see \cref{nested-for-loops-and-reduction-expressions-with-multiple-iterators} for more information:
 \begin{lstlisting}[language=grammar]
 for-indices:
    for-index {"," for-index}