Fix warning from :table_rex

[billylanchantin]

Fixes: #1063

No more warning:

iex(1)> df = Explorer.Datasets.iris()
iex(2)> Explorer.DataFrame.print(df)
+--------------------------------------------------------------------------+
|               Explorer DataFrame: [rows: 150, columns: 5]                |
+--------------+-------------+--------------+-------------+----------------+
| sepal_length | sepal_width | petal_length | petal_width |    species     |
|    <f64>     |    <f64>    |    <f64>     |    <f64>    |    <string>    |
+==============+=============+==============+=============+================+
| 5.1          | 3.5         | 1.4          | 0.2         | Iris-setosa    |
| 4.9          | 3.0         | 1.4          | 0.2         | Iris-setosa    |
| 4.7          | 3.2         | 1.3          | 0.2         | Iris-setosa    |
| …            | …           | …            | …           | …              |
| 6.2          | 3.4         | 5.4          | 2.3         | Iris-virginica |
| 5.9          | 3.0         | 5.1          | 1.8         | Iris-virginica |
+--------------+-------------+--------------+-------------+----------------+

:ok