fix(virtual indent): offset on_lines start & end by +1

This ensures the headlines grab the correct range, prior to this on
nightly (0.10.0) it would appear that the range of lines grabbed caused
the lookup to include the previous headline in the treesitter lookup.

By offsetting forward by 1, treesitter correctly grabs the nearest (new)
headline and sets the virtual indentation accordingly.

lua/orgmode/ui/virtual_indent.lua

@@ -148,7 +148,7 @@ function VirtualIndent:attach()
       end
 
       vim.schedule(function()
-        self:set_indent(start_line, end_line)
+        self:set_indent(start_line + 1, end_line + 1)
       end)
     end,
     on_reload = function()