Update _episodes/06-preprocessor.md

Co-authored-by: Rémi Kazeroni <[email protected]>

Author: mo-tgeddes

_episodes/06-preprocessor.md

@@ -120,7 +120,7 @@ In this case, it gives an error. Below you see the last few lines of the error m
 Error validating data /home/user/esmvaltool_tutorial/recipe_warming_stripes.yml
         with schema /home/user/mambaforge/envs/esmvaltool_tutorial/lib/python3.10
         /site-packages/esmvalcore/recipe_schema.yml
-	documentation.authors: Required field missing
+            documentation.authors: Required field missing
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