ox-latex: Fix nested markup in TODO keyword defalt format

* lisp/ox-latex.el (org-latex-format-headline-default-function): Use
  declarative shape to nest makup for TODO keywords.  Previous syntax
  generated errors during export.

lisp/ox-latex.el

@@ -1506,7 +1506,7 @@ holding contextual information."
   "Default format function for a headline.
 See `org-latex-format-headline-function' for details."
   (concat
-   (and todo (format "\\textbf{\\textsc{\\textsf{%s}}} " todo))
+   (and todo (format "{\\bfseries\\sffamily %s} " todo))
    (and priority (format "\\framebox{\\#%c} " priority))
    text
    (and tags