Fix javascript interaction with right-hand-side images in two-up diff views in Differential

Summary: Fixes T10704. This is just bad copy-paste -- "O" for "old" should be "N" for "new".

Test Plan:

• Followed steps on T10704.
• Applied patch.
• Marked inline done, replied, etc. No more JS errors.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10704

Differential Revision: https://secure.phabricator.com/D15566