Change underscores to hyphens in section labels

* lisp/org-exp.el (org-export-define-heading-targets): Use dash instead
of underscore to make labels valid.

Currently, the section number (a string of the form "X.Y.Z") is used
to manufacture a section label of the form sec-X_Y_Z. Underscores are
overloaded however, both in org and in LaTeX, and cause conflicts with
some packages.  This patch changes underscores to hyphens in section
labels.

TINYCHANGE

lisp/org-exp.el

@@ -1298,7 +1298,7 @@ Also find all ID and CUSTOM_ID properties and store them."
 				       (org-outline-level))))
 	  (setq target (org-solidify-link-text
 			(format "sec-%s" (replace-regexp-in-string
-					  "\\." "_"
+					  "\\." "-"
 					  (org-section-number level)))))
 	  (setq last-section-target target)
 	  (push (cons target target) target-alist)