Fix sticky table header on "Find Activities" page

[christianwach]

Overview

Fixes this issue on Lab.

The templates/CRM/Activity/Form/Selector.tpl template assigns the sticky class to the wrong element and therefore its sticky header is never initialised.

Before

No sticky table header on "Find Activities" page

After

Sticky table header on "Find Activities" page works as expected.

[civibot]

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[colemanw]

Checked the existing code and can confirm this one is an outlier. All others apply the class to <thead>.