[css-animations-2] Fix custom-ident to dashed-ident for animation-tim…

…eline

In #8746 it was resolved to switch from `<custom-ident>` to `<dashed-ident>` for timeline names, and this was done in the grammar in 9f39808. However, further down in the spec, it still shows `<custom-ident>`

css-animations-2/Overview.bs

@@ -654,7 +654,7 @@ the following effects:
 : <dfn>none</dfn>
 :: The animation is not associated with a [=timeline=].
 
-: <dfn><<custom-ident>></dfn>
+: <dfn><<dashed-ident>></dfn>
 :: If a named [=scroll progress timeline=] or [=view progress timeline=]
  is in scope on this element,
  use the referenced timeline