add exclude.

Author: BillWagner

docfx.json

@@ -78,6 +78,7 @@
                     "csharp-8.0/null-coalescing-assignment.md",
                     "csharp-8.0/async-streams.md",
                     "csharp-8.0/ranges.md",
+                    "csharp-8.0/using.md",
                     "csharp-9.0/nullable-reference-types-specification.md",
                     "csharp-9.0/nullable-constructor-analysis.md",
                     "csharp-9.0/nullable-parameter-default-value-analysis.md",