Update SenticGCN documentation to reflect paper training method call

[raymondng76]

See issues reported here, https://epoch.aisingapore.org/community/sg-nlp/how-do-we-train-senticgcn-models-with-our-dataset/

The documentation only listed the trainer code for executing the trainer function but did not indicate that the config file should be pass via the --config flag.

Update documentation to indicate how to pass the config file to the trainer.