[css-nesting-1] Fix linking of <<scope-start>> (#12991)

`<scope-start>` is a production, and should use the production
markup `<<scope-start>>`.

Fixes this error:

  LINE 1347:22: ''...'' shorthand (opened on 1347:7) was closed,
  but there were still open elements inside of it.

Author: andruud

css-nesting-1/Overview.bs

@@ -1344,7 +1344,7 @@ will collapse into a single rule when serialized and parsed again.
 Significant changes since the
 <a href="https://www.w3.org/TR/2023/WD-css-nesting-1-20230214/">Feb 14, 2023 Working Draft</a>:
 
-* The ''<scope-start>'' selector of a ''@scope'' rule
+* The <<scope-start>> selector of a ''@scope'' rule
 	no longer acts as the parent rule for nesting.
 	(<a href="https://github.com/w3c/csswg-drafts/issues/9740">Issue 9740</a>)