docs: fix image type to png (#2152)

Signed-off-by: xjasonlyu <xjasonlyu@gmail.com>

docs/en_US/usecases/public_data_analysis.md

@@ -160,7 +160,7 @@ requirements in various scenarios, even without using extended plugins.
 Since we already have the starting and ending coordinates of the bikes in our data,
 we can calculate the average speed of the bikes by applying the distance formula based on latitude and longitude:
 
-![coordinator](./resources/formula.webp)
+![coordinator](./resources/formula.png)
 
 The explanation of the formula is as follows:
 

docs/zh_CN/usecases/public_data_analysis.md

@@ -147,7 +147,7 @@ eKuiper 内置了丰富的 SQL 函数，即便在不是用插件拓展的情况
 
 我们的数据中已经有了单车的起始和结束坐标，所以我们可以根据经纬度计算距离公式，来计算出单车的平均速度：
 
-![coordinator](./resources/formula.webp)
+![coordinator](./resources/formula.png)
 
 其中：