Update README.md

Author: giovannivolpe

README.md

@@ -229,7 +229,7 @@ Specific examples for graph-neural-network-based particle linking and trace char
 
 - DTAT391A **[deeptrack.sources.base](https://github.com/DeepTrackAI/DeepTrack2/blob/develop/tutorials/3-advanced-topics/DTAT391A_sources.base.ipynb)** <a href="https://colab.research.google.com/github/DeepTrackAI/DeepTrack2/blob/develop/tutorials/3-advanced-topics/DTAT391A_sources.base.ipynb" target="_parent"><img src="https://colab.research.google.com/assets/colab-badge.svg" alt="Open In Colab"/></a>
 
-- DTAT391B **[deeptrack.sources.folder]**
+- DTAT391B **[deeptrack.sources.folder](https://github.com/DeepTrackAI/DeepTrack2/blob/develop/tutorials/3-advanced-topics/DTAT391B_sources.folder.ipynb)** <a href="https://colab.research.google.com/github/DeepTrackAI/DeepTrack2/blob/develop/tutorials/3-advanced-topics/DTAT391B_sources.folder.ipynb" target="_parent"><img src="https://colab.research.google.com/assets/colab-badge.svg" alt="Open In Colab"/></a>
 
 - DTAT391C **[deeptrack.sources.rng](https://github.com/DeepTrackAI/DeepTrack2/blob/develop/tutorials/3-advanced-topics/DTAT391C_sources.rng.ipynb)** <a href="https://colab.research.google.com/github/DeepTrackAI/DeepTrack2/blob/develop/tutorials/3-advanced-topics/DTAT391C_sources.rng.ipynb" target="_parent"><img src="https://colab.research.google.com/assets/colab-badge.svg" alt="Open In Colab"/></a>