fix docstring dispatch

[johnnychen94]

This fixes the following warning:

julia> using DitherPunk

julia> using Clustering
┌ Warning: Replacing docs for `DitherPunk.dither! :: Union{}` in module `DitherPunk`
└ @ Base.Docs docs/Docs.jl:240
┌ Warning: Replacing docs for `DitherPunk.dither :: Union{}` in module `DitherPunk`
└ @ Base.Docs docs/Docs.jl:240

help?> dither
search: dither dither! DitherPunk OrderedDither isdispatchtuple

  dither([T::Type,] img, alg::AbstractDither, ncolors; maxiter, tol, kwargs...)

  Dither image img using algorithm alg. A color palette with ncolors is computed by Clustering.jl's K-means clustering. The amount of
  maxiter and tolerance tol default to those exported by Clustering.jl.

After: two pieces of method docstring

julia> using DitherPunk

julia> using Clustering

help?> dither
search: dither dither! DitherPunk OrderedDither isdispatchtuple

  dither([T::Type,] img, alg::AbstractDither, args...; kwargs...)

  Dither image img using algorithm alg.

  Output
  ≡≡≡≡≡≡≡≡

  If no return type is specified, dither will default to the type of the input image.

  ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

  dither([T::Type,] img, alg::AbstractDither, ncolors; maxiter, tol, kwargs...)

  Dither image img using algorithm alg. A color palette with ncolors is computed by Clustering.jl's K-means clustering. The amount of
  maxiter and tolerance tol default to those exported by Clustering.jl.

[codecov[bot]]

Codecov Report

Merging #56 (ba205fd) into master (0cb8792) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #56   +/-   ##
=======================================
  Coverage   96.38%   96.38%           
=======================================
  Files          14       14           
  Lines         221      221           
=======================================
  Hits          213      213           
  Misses          8        8           

Impacted Files	Coverage Δ
src/clustering.jl	100.00% <ø> (ø)

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[adrhill]

Perfect, thanks!