lisp/org-table.el: Bugfix: correctly handle increment of -/+ numbers

* lisp/org-table.el (org-table-copy-down): Bugfix: correctly
handle increment of numbers with a -/+ sign.

lisp/org-table.el

@@ -1155,7 +1155,7 @@ to a number.  In the case of a timestamp, increment by days."
 			      (- (org-time-string-to-absolute txt)
 				 (org-time-string-to-absolute txt-up)))
 			     ((string-match org-ts-regexp3 txt) 1)
-			     ((string-match "^[0-9]+\\(\.[0-9]+\\)?" txt-up)
+			     ((string-match "\\([-+]\\)?[0-9]+\\(?:\.[0-9]+\\)?" txt-up)
 			      (- (string-to-number txt)
 				 (string-to-number (match-string 0 txt-up))))
 			     (t 1)))