Fix cypher for SSL error in ESP32-lwmqtt example

[Hiroya-W]

Hi.
As shown in #221, the current ESP32-lwmqtt example gives a cypher for SSL error.
In this PR, I will fix this problem by modifying root_cert and setting root_cert to WiFiClientSecure instance.
This solution is mentioned in #221 (comment) .

Thank you.

[E][WiFiClientSecure.cpp:133] connect(): start_ssl_client: -1
not connected
Settings incorrect or missing a cyper for SSL
Connect with mqtt.2030.ltsapis.goog:8883
ClientId: projects/***
Waiting 60 seconds, retry will likely fail

[gguuss]

Tested on ESP32, confirmed to work, merging.