Summary

Documentation about flow_from_dataframe does not render correctly. https://keras.io/preprocessing/image/
This PR solves the bug in the markdown.

PR Overview

• [n ] This PR requires new unit tests [y/n] (make sure tests are included)
• [ y] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [ y] This PR is backwards compatible [y/n]
• [ n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)