better link to 2021 materials

Author: haesleinhuepf

README.md

@@ -61,7 +61,7 @@ If you have any questions, please use the anonymous etherpad (see email) or open
 ## See also
 
 ### Former & future lecture materials
-* [Bio-image Analysis, programming, bio-statistics and machine learning 2021](https://github.com/BiAPoL/Bio-image_Analysis_with_Python/releases/tag/2021.08.03)
+* [Bio-image Analysis, programming, bio-statistics and machine learning 2021](https://github.com/BiAPoL/Bio-image_Analysis_with_Python/tree/a62070dee408814cee4258758f5187f135774519)
 * [Bio-image Analysis, programming, bio-statistics and machine learning 2020](https://git.mpi-cbg.de/rhaase/lecture_applied_bioimage_analysis_2020)
 * [Bio-image Analysis, ImageJ Macro programming 2019](https://git.mpi-cbg.de/rhaase/lecture_applied_bioimage_analysis)
 * [Bio-image Analysis Notebooks](https://haesleinhuepf.github.io/BioImageAnalysisNotebooks/)