Fix improper FFTW function deprecation

[ararslan]

I changed something incorrectly during a rebase in #21956. This now works as expected:

julia> fft(1)
ERROR: fft has been moved to the package FFTW.jl.
Run `Pkg.add("FFTW")` to install FFTW then run `using FFTW` to load it.
Stacktrace:
 [1] #fft#8(::Array{Any,1}, ::Function, ::Int64, ::Vararg{Int64,N} where N) at ./deprecated.jl:1457
 [2] fft(::Int64, ::Vararg{Int64,N} where N) at ./deprecated.jl:1457

[JeffBezanson]

The official way to handle this is you need to double-interpolate: "$($x)":

julia> x = 88
88

julia> :($x, "$x", "$($x)")
:((88, "$(x)", "$(88)"))

[ararslan]

Yep, just realized that and changed it here. Thanks!