Fixes missing org-html-format-headline--wrap error due to org-mode commi...

...t: c9ca0b6df86de13a5302b5a3c955fb2fb1023d36

Sorry if I'm missing something (I'm new to elisp) but it looks like org-mode renamed this function recently so if you use their latest version org-reveal export will not work. This fixes that error and is working for me on the newest version of org-mode.