use `bin/spring` in release notes. [ci skip]

Follow up to: https://github.com/rails/rails/commit/828a8f214535e59d709fd4862605902d1cc21632#commitcomment-4879462

This will be available after https://github.com/jonleighton/spring/commit/c6e25804b3338959d87a29f40967a333c650c031
is released.

guides/source/4_1_release_notes.md

@@ -101,7 +101,7 @@ bin/rails console
 **spring introspection:**
 
 ```
-$ bundle exec spring status
+$ bin/spring status
 Spring is running:
 
  1182 spring server | my_app | started 29 mins ago