Corrected error in description of Laplacian solver

Default value of coefficient a is 0, not 1 as given in manual.

Closes issue bendudson#37

manual/tex_files/user_manual.tex

@@ -3210,7 +3210,7 @@ \subsection{Laplacian inversion}
 \end{lstlisting}
 By default, this will use the options in a section called ``laplace'', but can be given
 a different section as an argument.
-By default $d = a = 1$ and the $c$ term is switched off. To set the values of these
+By default $d = 1$, $a = 0$, and the $c$ term is switched off. To set the values of these
 coefficients, there are the \lstinline!setCoefA()!, \lstinline!setCoefC()!, and \lstinline!setCoefD()!
 methods:
 \begin{lstlisting}