minor doc fix for r.md
minor doc fix for r.md

### What is this PR for?
straightforward doc fix

### What type of PR is it?
[Documentation]

### Todos
* [ ] - Task

### What is the Jira issue?
* No jira created as it is very trival.

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: Jeff Zhang <zjffdu@apache.org>

Closes #1268 from zjffdu/r_doc_fix and squashes the following commits:

105fead [Jeff Zhang] minor doc fix for r.md

(cherry picked from commit 6b6f1cb76cb2c9701a3accb3a4ee365d6566eb7a)
Signed-off-by: Mina Lee <bb9c4f68b7c04206772f92363630483486088f46@apache.org>

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