chart: fix showLeaderLines tag

PR #1020

Author: sebastian2010

xlsxwriter/chart.py

@@ -3928,7 +3928,7 @@ def _write_show_leader_lines(self):
 
         self._xml_start_tag("c:extLst")
         self._xml_start_tag("c:ext", attributes)
-        self._xml_empty_tag("<c15:showLeaderLines", [("val", 1)])
+        self._xml_empty_tag("c15:showLeaderLines", [("val", 1)])
         self._xml_end_tag("c:ext")
         self._xml_end_tag("c:extLst")