MAINT: fix accessor's doc

[Zeroto521]

Problem description

callable is a function to judge an object could call or not.
But I thought it was a function type.

Before

Now

[codecov]

Codecov Report

Merging #203 (cee5f92) into master (afb20dd) will decrease coverage by 1.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #203      +/-   ##
==========================================
- Coverage   99.37%   98.30%   -1.08%     
==========================================
  Files          22       20       -2     
  Lines         479      471       -8     
  Branches       50       49       -1     
==========================================
- Hits          476      463      -13     
- Misses          1        4       +3     
- Partials        2        4       +2     

Impacted Files	Coverage Δ
dtoolkit/util/__init__.py	100.00% <ø> (ø)
dtoolkit/accessor/register.py	100.00% <100.00%> (ø)
dtoolkit/transformer/factory.py	100.00% <100.00%> (ø)
dtoolkit/accessor/_util.py	76.47% <0.00%> (-23.53%)	⬇️
dtoolkit/accessor/dataframe.py	96.77% <0.00%> (-1.62%)	⬇️

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