fix typo

@graph/graph.m

@@ -145,7 +145,7 @@ assert(any(strcmp(o.shadeColor, valid_shadeColor)), ['@graph.graph: shadeColor m
         strjoin(valid_shadeColor)]);
 assert(any(strcmp(o.zeroLineColor, valid_shadeColor)), ...
        ['@graph.graph: zeroLineColor must be one of ' strjoin(valid_shadeColor)]);
-assert(any(strcmp(o.axisShape, {'box', 'L'})), ['@graph.graph: shadeColor ' ...
+assert(any(strcmp(o.axisShape, {'box', 'L'})), ['@graph.graph: axisShape ' ...
                     'must be one of ''box'' or ''L''']);
 valid_legend_locations = ...
     {'south west','south east','north west','north east','outer north east'};