changed br to double break line

Author: Fienne

src/introduction/prerequisites.md

@@ -55,7 +55,9 @@ $ (venv) pip install cwltool
 ```
 
 ```{note}
-You can find the `cwl-runner source code` [here](https://github.com/common-workflow-language/cwltool/tree/main/cwlref-runner). <br>
+You can find the `cwl-runner source code` [here](https://github.com/common-workflow-language/cwltool/tree/main/cwlref-runner).
+
+
 Visit the `cwltool` [documentation](https://github.com/common-workflow-language/cwltool#install)
 for other ways to install `cwltool` with `apt` and `conda`.
 ```