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

src/applications/diffusion/controller/DiffusionBrowseController.php

@@ -963,7 +963,7 @@ final class DiffusionBrowseController extends DiffusionController {
     }
 
     $header = id(new PHUIHeaderView())
-      ->setHeader(pht('Recently Open Revisions'));
+      ->setHeader(pht('Recent Open Revisions'));
 
     $list = id(new DifferentialRevisionListView())
       ->setViewer($viewer)