Fix broken link: Update Mask detection PyTorch Lightning tutorial URL

- Replace broken TowardsDataScience link that returned 404
- New link covers COVID-19 mask detection with PyTorch
- Fixes practical-tutorials#788

Author: Arshp-svg

README.md

@@ -501,7 +501,7 @@ To get started, simply fork this repo. Please refer to [CONTRIBUTING.md](CONTRIB
 - [Code a Smile Classifier using CNNS in Python](https://github.com/kylemcdonald/SmileCNN)
 - [Natural Language Processing using scikit-learn](https://towardsdatascience.com/natural-language-processing-count-vectorization-with-scikit-learn-e7804269bb5e)
 - [Code a Taylor Swift Lyrics Generator](https://towardsdatascience.com/ai-generates-taylor-swifts-song-lyrics-6fd92a03ef7e)
-- [Mask detection using PyTorch Lightning](https://towardsdatascience.com/how-i-built-a-face-mask-detector-for-covid-19-using-pytorch-lightning-67eb3752fd61)
+- [Mask detection using PyTorch Lightning](https://towardsdatascience.com/creating-a-powerful-covid-19-mask-detection-tool-with-pytorch-d961b31dcd45)
 
 ### Miscellaneous: