Fixed error with depreciated default in pandas>2

Author: catchSheep

episodes/02-starting-with-data.md

@@ -516,10 +516,10 @@ numeric data.
 # Summary statistics for all numeric columns by sex
 grouped_data.describe()
 # Provide the mean for each numeric column by sex
-grouped_data.mean()
+grouped_data.mean(numeric_only=True)
 ```
 
-`grouped_data.mean()` **OUTPUT:**
+`grouped_data.mean(numeric_only=True)` **OUTPUT:**
 
 ```output
         record_id     month        day         year    plot_id  \