Make various minor corrections to cheatsheet

[jbwhit]

Changes made:

• Fixed SubHeading Make New Variables to Make New Columns -- because these operations create new columns.
• Fixed # of distinct values in a column to use the pandas method .nunique() rather than len(df['w'].unique()).
• For clarity, renamed the Length column selection to w under the Summarize Data -- can see this being misleading to someone thinking that to get "Count number of rows with each value of the variable" they might need to call Length.
• For legibility/clarity removed the times that the example df was renamed (instances of df=df.dropna()); we should perhaps make a higher-level note that all of these operations are by default not done inplace, and either need to reassign or inplace=True.

Updated .pptx and .pdf

[codecov]

Codecov Report

Merging #16136 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #16136      +/-   ##
==========================================
- Coverage   90.86%   90.83%   -0.03%     
==========================================
  Files         159      159              
  Lines       50773    50773              
==========================================
- Hits        46134    46122      -12     
- Misses       4639     4651      +12

Flag	Coverage Δ
#multiple	88.61% <ø> (-0.03%)	⬇️
#single	40.33% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	63.54% <0%> (-1.82%)	⬇️

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[codecov]

Codecov Report

Merging #16136 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #16136      +/-   ##
==========================================
- Coverage   90.86%   90.83%   -0.03%     
==========================================
  Files         159      159              
  Lines       50773    50773              
==========================================
- Hits        46134    46122      -12     
- Misses       4639     4651      +12

Flag	Coverage Δ
#multiple	88.61% <ø> (-0.03%)	⬇️
#single	40.33% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	63.54% <0%> (-1.82%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 186957e...aaa04f3. Read the comment docs.

[jreback]

lgtm. @jorisvandenbossche

[jorisvandenbossche]

@jbwhit thanks!