docs: Changed "Chrome OS" to "ChromeOS" per current branding

Since 2022, the name of the OS has been "ChromeOS", not "Chrome OS" (see
the styling on https://www.google.com/chromebook/chrome-os/ )

NO_TEST , fixes a doc bug (no code edits).

Change-Id: I6da6120e1deeb57e7585690de41efc9f401bd699
Test: [go/abtd docs build]
Fixes: 281122893