docs: update to_dataframe sample to latest dependencies (#72)

Co-authored-by: Leah E. Cole <6719667+leahecole@users.noreply.github.com>

Author: tswast

samples/to_dataframe/requirements.txt

@@ -1,6 +1,6 @@
 google-auth==1.21.0
-google-cloud-bigquery-storage==1.0.0
-google-cloud-bigquery==1.25.0
+google-cloud-bigquery-storage==2.0.0
+google-cloud-bigquery==2.1.0
 pyarrow==1.0.1
 ipython==7.10.2; python_version > '3.0'
 ipython==5.9.0; python_version < '3.0'