Fix pm25.in typo

Small typo

README.md

@@ -306,7 +306,7 @@ API | Description | Auth | HTTPS | CORS |
 | [AirVisual](https://airvisual.com/api) | Air quality and weather data | `apiKey` | Yes | Unknown |
 | [GrünstromIndex](https://www.corrently.de/hintergrund/gruenstromindex/index.html) | Green Power Index for Germany (Grünstromindex/GSI) | No | No | Yes |
 | [OpenAQ](https://docs.openaq.org/) | Open air quality data | `apiKey` | Yes | Unknown |
-| [PM2.5.in](http://www.pm25.in/api_doc) | Air quality of China | `apiKey` | No | Unknown |
+| [PM25.in](http://www.pm25.in/api_doc) | Air quality of China | `apiKey` | No | Unknown |
 | [PVWatts](https://developer.nrel.gov/docs/solar/pvwatts/v6/) | Energy production photovoltaic (PV) energy systems | `apiKey` | Yes | Unknown |
 | [UK Carbon Intensity](https://carbon-intensity.github.io/api-definitions/#carbon-intensity-api-v1-0-0) | The Official Carbon Intensity API for Great Britain developed by National Grid | No | Yes | Unknown |