Update visual display of SHA in Rapid Diffs options menu

Context

In !204792 (merged), we changed "View file @ [SHA]" to "View file at [SHA]".

One small benefit to using the @ is it visually distinguishes readable text ("View file") from text that's not really human-readable ("[SHA]").

Problem

With the new text, the [SHA] sort of blends into the text and looks unusual alongside what's intended to be readable.

Before (master)	After (rapid_diffs=true)

Proposed Solution

Wrap the [SHA] in a pre/code tag to make it stand out from the readable text as "code".