Update PVWatts to v6 (#894)

According to the documentation, v5 is deprecated as of 12/2018. Update
it to v6 and also fix the link formatting (`-v5` previously redirected
to `/v5` but `-v6` does not redirect to `/v6`).

README.md

@@ -299,7 +299,7 @@ API | Description | Auth | HTTPS | CORS |
 | [AirVisual](https://airvisual.com/api) | Air quality and weather data | `apiKey` | Yes | Unknown |
 | [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 |
-| [PVWatts](https://developer.nrel.gov/docs/solar/pvwatts-v5/) | Energy production photovoltaic (PV) energy systems | `apiKey` | Yes | 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 |
 
 ### Events