Fixing mobile bert configuration doctest
Author: RamitPahwa
Pull request: yes

# What does this PR do?
Add configuration_mobilebert.py to utils/documentation_tests.txt for doctest.

Based on issue #19487

@sgugger / @ydshieh 
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@RamitPahwa There is an extra empty line in `configuration_mobilebert.py` that causes the tests to fail. 
It would work nicely once you remove it =)

@daspartho Thank for the help, should work now !

And thanks @daspartho for the help 💯