added quotes to NaN definition to make it clear

README.md

@@ -948,7 +948,7 @@ You can customize certain error messages when creating a nan schema.
 ```ts
 const isNaN = z.nan({
   required_error: "isNaN is required",
-  invalid_type_error: "isNaN must be a number",
+  invalid_type_error: "isNaN must be 'not a number'",
 });
 ```