[AIRFLOW-1678] Fix erroneously repeated word in function docstrings
[AIRFLOW-1678] Fix erroneously repeated word in function docstrings

Closes #2660 from thundergolfer/thundergolfer
/small-docstring-fix

mrares/incubator-airflow,mrares/incubator-airflow,mrares/incubator-airflow,mrares/incubator-airflow