Fix mg/m³ conversion formula for TVOC sensor

Updated the formula for converting ppb to mg/m³ for TVOC sensor.

Author: mash2k3

README.md

@@ -70,7 +70,7 @@ The integration supports automatic discovery of Qingping CGS1/CGS2/CGDN1 devices
 
 4. **TVOC Sensor**: The sensor can be set to 3 different measurement units, by default it is ppb. The component converts from ppb to get ppm and mg/m³.
    - ppm = ppb/1000
-   - mg/m³ = ppb/1000 * 0.0409 * 111.1 (concentration (ppm) x 0.0409 x molecular weight)<br />
+   - mg/m³ = ppb/218.77<br />
 
    **eTVOC Sensor**: The sensor can be set to 3 different measurement units, by default it is VOC index. The component converts from voc index to get ppb and mg/m³.
    - ppb = (index * 5) + 35