Update 04-recipe.md

Author: hb326

_episodes/04-recipe.md

@@ -173,7 +173,7 @@ Please note the following sections:
 
   - preprocessors: lines 25-28
 
-    The definition for different preprocessors or combinations. If no
+    The definition for different preprocessors or preprocessor combinations. If no
     preprocessing is needed, the preprocessor can be set to an empty python
     dictionary (`{}`). Here, we produce annual means. The preprocessor is called
     with its name (here: prep_timeseries), later in the diagnostic (line 39).