[SPARKR][MINOR] Add Xiangrui and Felix to maintainers

## What changes were proposed in this pull request?

This change adds Xiangrui Meng and Felix Cheung to the maintainers field in the package description.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #14758 from shivaram/sparkr-maintainers.