Typo: 'exiting' might be 'existing'

[ngolin]

https://github.com/reactjs/rfcs/blob/7f8492f6a177fc33fe807d242319f2f96353bf68/text/0188-server-components.md?plain=1#L334C163-L334C170

[rahul9852-dot]

Hi @lihz6
If no one is working on the above issue. Can i contribute on the above issues

[rahul9852-dot]

@lihz6 Fixed the above spelling mistake. Created PR for that as well if everything works fine. Please merge it.
It would be my first contribution.