Fixes a typo in the documentation (slwf_centres)

doc/user_guide/parameters.tex

@@ -1153,10 +1153,10 @@ \subsection{Constraints on centres}
 
 The block below shows an example of how to set the constraints:
 
-\noindent \verb#begin centre_constraints# \\
+\noindent \verb#begin slwf_centres# \\
 \verb#   2  0.0   0.0  0.0# \\
 \verb#   4  0.25  0.0  0.0# \\
-\verb#end centre_constraints#
+\verb#end slwf_centres#
 
 \begin{itemize}
 \item The first line sets the constraint for the centre of objective WF number 2 (as defined by the order of WFs in the {\tt projections} block) to (0.0,0.0,0.0) in fractional co-ordinates.