Make various minor corrections to cheatsheet
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