[Slotted options] Add backticks to select in link to keep it from eating the whole doc (#1370)

The "`<select>`" in the link to the new explainer was being treated as
an actual `<select>` by the markdown parser resulting in a mangled
document. Fix it by adding backticks.

Author: dandclark

SlottedOption/explainer.md

@@ -21,7 +21,7 @@ to keep the community up to date.
 
 - This document status: **Archived**
 - Expected venue: [WHATWG HTML](https://html.spec.whatwg.org/)
-- Current version: [Slotted Options in <select> - OpenUI](https://open-ui.org/components/select.slotted.option/)
+- Current version: [Slotted Options in `<select>` - OpenUI](https://open-ui.org/components/select.slotted.option/)
 
 ## Table of Contents