Recognize underscores in URL

* org.el (org-make-link-regexps): modified regexp of org-plain-link-re.

lisp/org.el

@@ -4853,7 +4853,7 @@ This should be called after the variable `org-link-types' has changed."
 	org-plain-link-re
 	(concat
 	 "\\<\\(" (mapconcat 'regexp-quote org-link-types "\\|") "\\):"
-	 (org-re "\\([^ \t\n()<>]+\\(?:([[:word:]0-9]+)\\|\\([^[:punct:] \t\n]\\|/\\)\\)\\)"))
+	 (org-re "\\([^ \t\n()<>]+\\(?:([[:word:]0-9_]+)\\|\\([^[:punct:] \t\n]\\|/\\)\\)\\)"))
 	;;	 "\\([^]\t\n\r<>() ]+[^]\t\n\r<>,.;() ]\\)")
 	org-bracket-link-regexp
 	"\\[\\[\\([^][]+\\)\\]\\(\\[\\([^][]+\\)\\]\\)?\\]"