DOC: Fix SS06 formatting errors in merge_asof docstrings
Errors fixed (from the list in #29254 ):
pandas.merge_asof

Validated the fixes with python scripts/validate_docstrings.py
Refrencing issue: #29254

Fix : pandanistas/pandanistas_sprint_ui2020#11
Hello @NewbieWorks! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-02-29 05:08:01 UTC Thanks @NewbieWorks 