Fix mamba env create example

slides.qmd

@@ -169,7 +169,7 @@ This makes the environment easier to reproduce, to test, and to maintain.
 
 * Interactively create and populate a virtual environment
   ```{raw}
-  mamba create -n my_env python=3.12
+  mamba create -n my_env -c conda-forge python=3.12
   mamba activate my_env
   mamba install matplotlib numpy scipy eccodes
   ```