Single device provisioning -> error on saving certificate command syntax

[iakko]

In the saving certificate command of this section, there is a syntax error on this command:

echo -e [CERTIFICATE_PEM] > $THING_NAME.crt

It should be:

echo -e '[CERTIFICATE_PEM]' > $THING_NAME.crt

Missing quotes creates a wrong pem format, not recognized in the mosquitto_pub command.