docs: use head() to get top n results, not to preview results (#190)

head() requires ordering. Just peeking at the whole DataFrame or Series is actually more efficient since it doesn't require ordering and still only downloads a fraction of the results.

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Fixes #<issue_number_goes_here> 🦕

Author: tswast

samples/snippets/pandas_methods_test.py

@@ -22,13 +22,20 @@ def test_bigquery_dataframes_pandas_methods():
     bq_df = bpd.read_gbq(query_or_table)
 
     # Inspect one of the columns (or series) of the DataFrame:
-    bq_df["body_mass_g"].head(10)
+    bq_df["body_mass_g"]
 
     # Compute the mean of this series:
     average_body_mass = bq_df["body_mass_g"].mean()
     print(f"average_body_mass: {average_body_mass}")
 
-    # Calculate the mean body_mass_g by species using the groupby operation:
-    bq_df["body_mass_g"].groupby(by=bq_df["species"]).mean().head()
+    # Find the heaviest species using the groupby operation to calculate the
+    # mean body_mass_g:
+    (
+        bq_df["body_mass_g"]
+        .groupby(by=bq_df["species"])
+        .mean()
+        .sort_values(ascending=False)
+        .head(10)
+    )
     # [END bigquery_dataframes_pandas_methods]
     assert average_body_mass is not None