csv writer doc example mistype

[kingeasternsun]

Which issue does this PR close?

Closes #903

Rationale for this change

document error

What changes are included in this PR?

fix document error

Are there any user-facing changes?

no

[alamb]

Thank you @kingeasternsun

[alamb]

Integration test failure https://github.com/apache/arrow-rs/runs/4096796177?check_suite_focus=true is unrelated to changes in this PR

[codecov-commenter]

Codecov Report

Merging #904 (3929368) into master (2cf3178) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #904   +/-   ##
=======================================
  Coverage   82.30%   82.30%           
=======================================
  Files         168      168           
  Lines       48029    48029           
=======================================
+ Hits        39529    39531    +2     
+ Misses       8500     8498    -2     

Impacted Files	Coverage Δ
arrow/src/csv/writer.rs	82.05% <ø> (ø)
arrow/src/array/transform/mod.rs	85.33% <0.00%> (+0.13%)	⬆️
arrow/src/datatypes/datatype.rs	65.36% <0.00%> (+0.43%)	⬆️

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[alamb]

Integration test failure is unrelated to this PR

[kingeasternsun]

Thank you @kingeasternsun

my pleasure 😄