update servo infomation

docs/en/accessory/sg90_servo.md

@@ -11,7 +11,7 @@
 ## Description
 
 **SG90 servo** is a custom 9g servo motor from M5Stack. This is totally compatible with the "8Servos" HAT and "PuppyC" base. This micro servo is very easy to use for beginners. There is no need to use a dedicated servo-drive board with feedback control and a complicated gearbox. Any common pwm driver or a microcontroller can easily control this servo. Compared to driving a stepper motor, a servo is much simpler to use. You can use it to do interesting small projects requiring angular movement and/or control.
- The commonly used pulse width is 1-2ms (-90° position - 1ms , 0° position - 1.5ms, 90° position - 2ms). Going beyond the specified pulse width range will damage the servo.If you feel that the cable is not long enough, you can purchase the servo extension cable to extend it.
+ The commonly used pulse width is 1-2ms (-90° position - 1ms , 0° position - 1.5ms, 90° position - 2ms)，working at 50Hz. Going beyond the specified pulse width range will damage the servo.If you feel that the cable is not long enough, you can purchase the servo extension cable to extend it.
  The yellow wire is for the control signal, brown is for the ground, and the red wire is for the Vcc (voltage). Pay attention to the supply current and voltage to prevent the servo from getting damaged.
 
 ### Product Features

docs/zh_CN/accessory/sg90_servo.md

@@ -11,7 +11,7 @@
 ## 描述
 
 **SG90 servo** 是一款M5Stack定制的9g舵机.旋转角度为0~180°，可以配合8Servos HAT 和 PuppyC 使用.这款小舵机对于初学者来说非常容易使用，你不必使用专用带有反馈控制的驱动板和复杂的减速箱，
-任何常见的舵机驱动程序和单片机都能轻易的让它转起来.相比驱动步进电机，使用它确实简单多了，你可以利用他来做一些有趣的小项目，控制一些角度.通常使用的脉冲宽度在1-2ms，0°位置（1.5ms），
+任何常见的舵机驱动程序和单片机都能轻易的让它转起来.相比驱动步进电机，使用它确实简单多了，你可以利用他来做一些有趣的小项目，控制一些角度.工作频率50Hz，通常使用的脉冲宽度在1-2ms，0°位置（1.5ms），
 -90°位置（1ms），90°位置（2ms）超出脉宽范围会使舵机损坏.如果你觉得连接线不够长可以购买舵机延长线将它延长，黄色为信号线，棕色为地线，红色为5V，使用时注意供电电流和电压防止被烧毁.