docs: clarify 'Table.read_rows' snippet

docs/snippets_table.py

@@ -202,14 +202,15 @@ def test_bigtable_write_read_drop_truncate():
 
     # Read full table
     partial_rows = table.read_rows()
-    read_rows = [row for row in partial_rows]
 
     # Read row's value
-    for row in read_rows:
+    total_rows = []
+    for row in partial_rows:
         cell = row.cells[COLUMN_FAMILY_ID][col_name][0]
         print(cell.value.decode("utf-8"))
+        total_rows.append(cell)
     # [END bigtable_read_rows]
-    assert len(read_rows) == len(rows)
+    assert len(list(total_rows)) == len(rows)
     # [START bigtable_drop_by_prefix]
     from google.cloud.bigtable import Client