Fix use_readme_rmd() to exclude comma after last argument passed to knitr::ops_chunk$set

It is this:

```r
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  fig.path = "man/figures/README-",
  out.width = "100%",  # <<<<<<<<<<<<<<<< This comma shouldn't be here.
)
```

It should be this:

```r
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  fig.path = "man/figures/README-",
  out.width = "100%"
)
```
