DOC: correct merge_asof example

[ZhuBaohe]

• closes DOC: Incorrect example result in 'merge_asof' docstring #19489

• tests added / passed

• passes git diff upstream/master -u -- "*.py" | flake8 --diff

• whatsnew entry

• update of incorrect documentation example for merge_asof.

• add a test corresponding to this example.

[pep8speaks]

Hello @ZhuBaohe! Thanks for updating the PR.

Cheers ! There are no PEP8 issues in this Pull Request. 🍻

Comment last updated on February 18, 2018 at 17:46 Hours UTC

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c4770c7). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #19737   +/-   ##
=========================================
  Coverage          ?   91.58%           
=========================================
  Files             ?      150           
  Lines             ?    48862           
  Branches          ?        0           
=========================================
  Hits              ?    44750           
  Misses            ?     4112           
  Partials          ?        0

Flag	Coverage Δ
#multiple	89.95% <ø> (?)
#single	41.75% <ø> (?)

Impacted Files	Coverage Δ
pandas/core/reshape/merge.py	94.25% <ø> (ø)

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[jreback]

thanks!