ADD: Update Air Speed Sensor SDF Element link in Readme (#554)

Signed-off-by: Ching-Hsiang Wu <tiger871108@gmail.com>
(cherry picked from commit 1fc25c4)

Author: TigerWuu

README.md

@@ -48,7 +48,7 @@ models into sensor streams.
 | **Sensor Name**           | **API Link**                                                                                                 | **SDF Element**                                                                                      | **Notes**                                          |
 |---------------------------|-------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------|----------------------------------------------------|
 | **Air Pressure Sensor**   | [AirPressureSensor](https://gazebosim.org/api/sensors/9/classgz_1_1sensors_1_1AirPressureSensor.html)        | [`<air_pressure>`](http://sdformat.org/spec?ver=1.12&elem=sensor#sensor_air_pressure)                | Measures atmospheric pressure                      |
-| **Air Speed Sensor**      | [AirSpeedSensor](https://gazebosim.org/api/sensors/9/classgz_1_1sensors_1_1AirSpeedSensor.html)              | N/A                                                                                    | Measures the speed of air relative to the sensor    |
+| **Air Speed Sensor**      | [AirSpeedSensor](https://gazebosim.org/api/sensors/9/classgz_1_1sensors_1_1AirSpeedSensor.html)              | [`<air_speed>`](http://sdformat.org/spec?ver=1.12&elem=sensor#sensor_air_speed)                                                                                  | Measures the speed of air relative to the sensor    |
 | **Altimeter**             | [AltimeterSensor](https://gazebosim.org/api/sensors/9/classgz_1_1sensors_1_1AltimeterSensor.html)            | [`<altimeter>`](http://sdformat.org/spec?ver=1.12&elem=sensor#sensor_altimeter)                      | Measures altitude above a reference point          |
 | **Bounding Box Camera**   | [BoundingBoxCameraSensor](https://gazebosim.org/api/sensors/9/classgz_1_1sensors_1_1BoundingBoxCameraSensor.html) | [`<camera>`](http://sdformat.org/spec?ver=1.12&elem=sensor#sensor_camera)                         | Captures images with bounding box annotations      |
 | **Camera**                | [CameraSensor](https://gazebosim.org/api/sensors/9/classgz_1_1sensors_1_1CameraSensor.html)                  | [`<camera>`](http://sdformat.org/spec?ver=1.12&elem=sensor#sensor_camera)                           | Captures standard RGB images                       |