doc: add missing title IsBigInt (#1352)

MarxJiao · web-flow · commit dfdf6eb6e68e · 2023-07-21T00:58:53.000+08:00
diff --git a/doc/value.md b/doc/value.md
@@ -141,6 +141,8 @@ bool Napi::Value::IsArrayBuffer() const;
 Returns `true` if the underlying value is a JavaScript `Napi::ArrayBuffer` or
 `false` otherwise.
 
+### IsBigInt
+
 ```cpp
 bool Napi::Value::IsBigInt() const;
 ```
