fix typo

src/@report/addSeries.m

@@ -35,7 +35,7 @@ assert(length(o.pages) > 0, ...
 assert(length(o.pages{end}.sections) > 0, ...
        ['@report.addSeries: Before adding a series, you must add a section and ' ...
         'either a graph or a table']);
-assert(length(o.pages{end}.sections.elements) > 0, ...
+assert(length(o.pages{end}.sections{end}.elements) > 0, ...
        ['@report.addSeries: Before adding a series, you must add ' ...
         'either a graph or a table']);
 assert(isa(o.pages{end}.sections{end}.elements{end}, 'graph') || ...