UpsidePotentialRatio missing r when MAR is time series

`UpsidePotentialRatio` fails when MAR is specified as time series:

```
> UpsidePotentialRatio(managers, MAR = managers$`US 10Y TR`)
Error in FUN(newX[, i], ...) : object 'r' not found
```

The issue is due to returns being specified as `r` instead of `R`
https://github.com/braverock/PerformanceAnalytics/blob/49a93f1ed6e2e159b63bf346672575f3634ed370/R/UpsidePotentialRatio.R#L91C31-L91C31
