Fix prompt in bookmark.el (Bug#24726)

* lisp/bookmark.el (bookmark-set-internal): Conform to the standard
default prompt format (per `minibuffer-electric-default-mode') which
does not use a colon.

lisp/bookmark.el

@@ -802,7 +802,7 @@ is ever deleted."
          (let ((str
                 (or name
                     (read-from-minibuffer
-                     (format "%s (default: \"%s\"): " prompt default)
+                     (format "%s (default \"%s\"): " prompt default)
                      nil
                      bookmark-minibuffer-read-name-map
                      nil nil defaults))))