fix build warning.

Author: BillWagner

docfx.json

@@ -56,7 +56,8 @@
                     "csharp-13.0/*.md",
                     "field-keyword.md",
                     "unbound-generic-types-in-nameof.md",
-                    "first-class-span-types.md"
+                    "first-class-span-types.md",
+                    "lambda-parameters-with-modifiers.md"
                 ],
                 "src": "_csharplang/proposals",
                 "dest": "csharp/language-reference/proposals",