Update python_virtual_environments.md: insert missing line. When executing...

Update python_virtual_environments.md: insert missing line. When executing this example, there is a conda init error raised if this line was not executed previously.

doc.zih.tu-dresden.de/docs/software/python_virtual_environments.md

@@ -207,6 +207,7 @@ can deactivate the conda environment as follows:
     [...]
     Proceed ([y]/n)? y
     [...]
+    marie@alpha$ source $EBROOTANACONDA3/etc/profile.d/conda.sh
     marie@alpha$ conda activate my-torch-env
     (my-torch-env) marie@alpha$ conda install -c pytorch torchvision
     Collecting package metadata (current_repodata.json): done