Update field-preview.md (#43583)

Author: jamesmontemagno

docfx.json

@@ -53,7 +53,8 @@
                     "csharp-10.0/*.md",
                     "csharp-11.0/*.md",
                     "csharp-12.0/*.md",
-                    "csharp-13.0/*.md"
+                    "csharp-13.0/*.md",
+                    "field-keyword.md"
                 ],
                 "src": "_csharplang/proposals",
                 "dest": "csharp/language-reference/proposals",