Update pandas.md

Fix: Handled missing values in numeric columns by using numeric_only=True in mean() to avoid errors from non-numeric columns.

chapter_preliminaries/pandas.md

@@ -55,7 +55,7 @@ print(data)
 ```{.python .input}
 #@tab all
 inputs, outputs = data.iloc[:, 0:2], data.iloc[:, 2]
-inputs = inputs.fillna(inputs.mean())
+inputs = inputs.fillna(inputs.mean(numeric_only=1))
 print(inputs)
 ```