adjust the name input of the SubSection to the new HTML structure

[kareemadel]

Bug Fix

Proposed changes

The code was crashing on line 391 of the file edx_dl/parsing.py
AttributeError: 'NoneType' object has no attribute 'string'. It was because the section name path changed from s.a.div.span to s.a.div.div, so I changed it accordingly.
#514

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have read the CONTRIBUTING doc
• I agree to contribute my changes under the project's LICENSE
• I have checked that the unit tests pass locally with my changes
• I have checked the style of the new code (lint/pep).
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[coveralls]

Coverage remained the same at 48.016% when pulling 933e1f7 on kareemadel:master into 55f6e51 on coursera-dl:master.

[rbrito]

Thank you very much, @kareemadel. I apparently merged another pull request that contained the same fix as yours.

I appreciate all the attention and interest that you have in edx-dl and, if you find other issues with the code, please do send other pull requests.

Thank you once again!