doc: clarify repl exception conditions

The sample code demonstrating ERR_INVALID_REPL_INPUT is confusing, I
think. This simplifies and clarifies it a bit, I hope.

PR-URL: https://github.com/nodejs/node/pull/37142


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>