Fix small typos in code-of-conduct.md

This includes a few minor typo fixes:
- Change 'on' to 'of' on line 14.
- Change 'at' to 'are' on line 12.
- Add a period to end of list on line 10.
Merge pull request #5 from mragray/patch-1

Fix small typos in code-of-conduct.md