Improve alt text for main logo
Improve alt text for main logo

Summary:
It's helpful to provide more context that specifically the image is the Watchman logo.

https://sc.edu/about/offices_and_divisions/digital-accessibility/guides_tutorials/alternative_text/logo-alt-text/index.php

Reviewed By: fanzeyi

Differential Revision: D30490472

fbshipit-source-id: 301279f59ad3c5af45cb68c30b56ab2d835526f9

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