Add CS8002.md to the language reference TOC.

This was missed in #43728

Noticed in #43805

Author: BillWagner

docs/csharp/language-reference/toc.yml

@@ -1949,6 +1949,8 @@ items:
       href: ./compiler-messages/cs8422.md
     - name: CS8515
       href: ./compiler-messages/cs8515.md
+    - name: CS8802
+      href: ./compiler-messages/cs8802.md
     - name: CS8803
       href: ./compiler-messages/cs8803.md
     - name: CS8812