R4F: Fix typos and something outdated in CONTRIBUTING.md

[yangyanqing]

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry: sdkch add [section] [stanza] [message]

• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #4132 into develop will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop   #4132      +/-   ##
==========================================
- Coverage    60.12%   60.1%   -0.02%     
==========================================
  Files          211     211              
  Lines        15138   15138              
==========================================
- Hits          9101    9099       -2     
- Misses        5415    5417       +2     
  Partials       622     622

[alexanderbez]

One small nit, otherwise LGTM. Thank you.