fix: compass

docs/api/system/compass.md

@@ -13,9 +13,10 @@
 **示例**
 
 ```javascript
-uni.onCompassChange(function (res) {
+const callback = function (res) {
 	console.log(res.direction);
-});
+}
+uni.onCompassChange(callback);
 ```
 
 ### uni.offCompassChange(CALLBACK)
@@ -27,6 +28,16 @@ uni.onCompassChange(function (res) {
 |:-:|:-:|:-:|:-:|:-:|:-:|:-:|:-:|:-:|
 |HBuilderX 2.9.6+|HBuilderX 2.9.6+|基础库 2.9.3+|x|x|x|x|√|x|
 
+**示例**
+```javascript
+const callback = function (res) {
+	console.log(res.direction);
+}
+uni.onCompassChange(callback);
+// 和 onCompassChange 传入同一个函数即可
+uni.offCompassChange(callback);
+```
+
 **Tips**
 - `CALLBACK`为调用`uni.onCompassChange`时传入的`CALLBACK`