DOC: Correct groupby().mean() usage in table layout getting started article (#55600)

* DOC: Correct groupby().mean() usage in table layout getting started article

* DOC: Explicitly select column in table layout article's groupby().mean() usage

Author: ragibson

doc/source/getting_started/intro_tutorials/07_reshape_table_layout.rst

@@ -266,7 +266,7 @@ For more information about :meth:`~DataFrame.pivot_table`, see the user guide se
 
     ::
 
-        air_quality.groupby(["parameter", "location"]).mean()
+        air_quality.groupby(["parameter", "location"])[["value"]].mean()
 
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