Use Series' scol instead of anthor DataFrame's first scol

[HyukjinKwon]

Small folowup of #704

[softagram-bot]

Softagram Impact Report for pull/707 (head commit: d773c50)

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[codecov-io]

Codecov Report

Merging #707 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #707      +/-   ##
==========================================
- Coverage   93.86%   93.86%   -0.01%     
==========================================
  Files          32       32              
  Lines        5545     5544       -1     
==========================================
- Hits         5205     5204       -1     
  Misses        340      340

Impacted Files	Coverage Δ
databricks/koalas/plot.py	94.73% <100%> (-0.02%)	⬇️

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