doc: corrects reference to tlsClientError

After the renaming of `clientError` in TLS to `tlsClientError` in
https://github.com/nodejs/node/commit/1ab6b21360d97719b3101153fb164c0c3eddf08,
the docs inconsistently referred the error as `clientError`, which is now
corrected.

Fixes: https://github.com/nodejs/node/issues/13417
PR-URL: https://github.com/nodejs/node/pull/13533


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>