Deprecated syntax in `/programming/computing/dataframes.md`

[DilumAluthge]

The relevant code is on lines 286-289 of https://github.com/bcbi/bidss/blob/master/programming/computing/dataframes.md

bidss/programming/computing/dataframes.md

Lines 286 to 289 in 356cc73

	```julia:./dataframes.jl
	# get counts of males and females in the dataframe
	println(by(df, :gender, N = :gender => length))
	```

Here is the deprecation warning: (you may need to scroll right to read the full warning)

┌ Warning: `by(d::AbstractDataFrame, cols::Any; sort::Bool = false, skipmissing::Bool = false, f...)` is deprecated, use `combine(groupby(d, cols, sort = sort, skipmissing = skipmissing), [if in_col isa ColumnIndex
│             in_col
│         else
│             AsTable(in_col)
│         end => (fun => out_col) for (out_col, (in_col, fun)) = f]...)` instead.
│   caller = ip:0x0
└ @ Core :-1