fix `rtn.rda` type and documentation

[dan9401]

I'm in the process of updating documentation regarding #17 , and encountered a roxygen issue due to the GSPC dataset name.

In short, the loaded object name by data(rtn) needs to be the same name as last line in data.R.

• [x] decide if we want to use rtn or gspc
• [x] Change file name, object name and documentation accordingly
• [x] Add explanation of ^GSPC ticker
• [x] Change rtn data type to be xts? (or am I missing something)

I would suggest to create a branch from 17-change-class-name-to-robustgarch (d1b80de) to finish this and merge it back to 17-change-class-name-to-robustgarch.

[EchoRLiu]

a small idea: should we update the data with the most current data? perhaps with more years? currently it is around 13 years

[dan9401]

Good point, let's do that, how did you source it last time?