Fix custom transforms docs example

[EssamWisam]

Last time when I worked on the docs, I misused Tables.materialize without noticing. When I ran the example, for some reason, Tables.jl did not return an error and instead returned the original table and I didn't notice that anything is wrong because I didn't focus on the specific numbers.

This issue is solved in the PR.

[codecov-commenter]

Codecov Report

Merging #191 (c2b365b) into master (d2334a1) will increase coverage by 0.18%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #191      +/-   ##
==========================================
+ Coverage   92.22%   92.40%   +0.18%     
==========================================
  Files          31       31              
  Lines        1067     1067              
==========================================
+ Hits          984      986       +2     
+ Misses         83       81       -2     

see 1 file with indirect coverage changes