Remove outdated safety example from readme

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

With the release of arrow 6.4.0 the example unsafe code in the readme is no longer possible, so remove it

What changes are included in this PR?

Remove outdated safety example from readme

Are there any user-facing changes?

Doc updates only

[codecov-commenter]

Codecov Report

Merging #1050 (2176f3e) into master (2d28534) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 2176f3e differs from pull request most recent head eceb0d6. Consider uploading reports for the commit eceb0d6 to get more accurate results

@@            Coverage Diff             @@
##           master    #1050      +/-   ##
==========================================
- Coverage   82.31%   82.31%   -0.01%     
==========================================
  Files         168      168              
  Lines       49056    49056              
==========================================
- Hits        40382    40378       -4     
- Misses       8674     8678       +4     

Impacted Files	Coverage Δ
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (-0.46%)	⬇️
arrow/src/array/transform/mod.rs	84.96% <0.00%> (-0.28%)	⬇️
parquet/src/encodings/encoding.rs	93.52% <0.00%> (-0.20%)	⬇️
arrow/src/datatypes/field.rs	53.68% <0.00%> (+0.30%)	⬆️

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