Bug in the example of DataFramesMeta doc.

[zdlspace0528]

df = DataFrame(x = [1, 1, 2, 2], y = [1, 2, 101, 102]);
@select(df, :x2 = :x, :x3 = ^(:x))
ERROR:
LoadError: ArgumentError: This path should not be reached
Stacktrace:
 [1] fun_to_vec(ex::Expr; nolhs::Bool, gensym_names::Bool, wrap_byrow::Bool)
   @ DataFramesMeta ~/.julia/packages/DataFramesMeta/z0mc6/src/parsing.jl:302
 [2] (::DataFramesMeta.var"#30#31"{Bool})(ex::Expr)
   @ DataFramesMeta ./none:0
 [3] iterate(::Base.Generator{Vector{Any}, DataFramesMeta.var"#30#31"{Bool}})
   @ Base ./generator.jl:47
 [4] select_helper(::Symbol, ::Expr, ::Vararg{Expr})
   @ DataFramesMeta ~/.julia/packages/DataFramesMeta/z0mc6/src/macros.jl:976
 [5] var"@select"(__source__::LineNumberNode, __module__::Module, x::Any, args::Vararg{Any})
   @ DataFramesMeta ~/.julia/packages/DataFramesMeta/z0mc6/src/macros.jl:1078
in expression starting at REPL[3]:1

[pdeffebach]

Apologies for being so long delayed in fixing this.

I have a PR #347 which fixed this problem.

[pdeffebach]

Fixed in #347