Don't advertise obsolete same-window-regexps

Since the user option same-window-regexps is provided only for
backward compatibility, rewrite the example in the README in terms
of the preferred display-buffer-alist, as per
(info "(elisp) Choosing Window Options").

README.md

@@ -153,7 +153,8 @@ If you prefer to roll out your own Elisp, you could add to your
 `user-init-file` something as simple as:
 
 ```el
-(add-to-list 'same-window-regexps "\\`\\*hackernews .*\\*\\'")
+(push '("\\`\\*hackernews .*\\*\\'" (display-buffer-same-window))
+      display-buffer-alist)
 ```
 
 ### Troubleshooting