[core] Fix spacemacs/alternate-buffer for other windows

According to its docstring, `(spacemacs/alternate-buffer WINDOW)` should switch
between the current and last buffer in WINDOW. Previously, it switched between
the (window-buffer WINDOW) and the last buffer of the selected window, which
does not seem intended.

core/core-funcs.el

@@ -347,7 +347,7 @@ only switches between the current layout's buffers."
                       (and (not (eq buffer my-buffer))
                            (funcall usefulp buffer)
                            (funcall predicate buffer))))
-                  (window-prev-buffers)
+                  (window-prev-buffers window)
                   default))
     (if (not buf)
         (message "Last buffer not found.")