[SPARK-27262][R] Add explicit UTF-8 Encoding to DESCRIPTION

## What changes were proposed in this pull request?

I got this warning when following the recommended approach to generating documentation:

```
Warning message:
roxygen2 requires Encoding: UTF-8
```

As can be seen in [other](https://github.com/tidyverse/tidyverse/blob/master/DESCRIPTION) [`tidyverse`](https://github.com/tidyverse/dplyr/blob/master/DESCRIPTION) [`DESCRIPTION`s](https://github.com/tidyverse/readr/blob/master/DESCRIPTION), this is standard practice

This PR adds `Encoding: UTF-8` to `R/pkg/DESCRIPTION`

## How was this patch tested?

Pass the Jenkins without warning.

Closes apache#23823 from MichaelChirico/patch-1.

Authored-by: Michael Chirico <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: MichaelChirico

R/pkg/DESCRIPTION

@@ -60,4 +60,5 @@ Collate:
     'window.R'
 RoxygenNote: 5.0.1
 VignetteBuilder: knitr
+Encoding: UTF-8
 NeedsCompilation: no