Retitle "Recently Open Revisions" panel to "Recent Open Revisions"

Summary:
Ref T13639. In D17754, this:

OPEN REVISIONS
Recently updated open revisions affecting this file.

...was simplified into:

RECENTLY OPEN REVISIONS

This is a bit misleading, since the panel doesn't contain "recently open" results. Use "Recent Open" instead, which is a bit more consistent with other product text. This is still slightly misleading, but probably close enough.

Test Plan: Read text.

Maniphest Tasks: T13639

Differential Revision: https://secure.phabricator.com/D21612