ARROW-11959: [Rust][DataFusion] Fix log line

[Dandandan]

Just a small fix to fix a debug line. It was printing the unoptimized plan instead of the optimized plan.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11959

[codecov-io]

Codecov Report

Merging #9699 (eb78919) into master (6a13206) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #9699   +/-   ##
=======================================
  Coverage   82.57%   82.57%           
=======================================
  Files         248      248           
  Lines       58279    58279           
=======================================
+ Hits        48121    48122    +1     
+ Misses      10158    10157    -1     

Impacted Files	Coverage Δ
rust/datafusion/src/execution/context.rs	90.84% <100.00%> (ø)
rust/parquet/src/encodings/encoding.rs	95.05% <0.00%> (+0.19%)	⬆️

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[alamb]

Thanks @Dandandan !

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.57%. Comparing base (6a13206) to head (eb78919).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9699   +/-   ##
=======================================
  Coverage   82.57%   82.57%           
=======================================
  Files         248      248           
  Lines       58279    58279           
=======================================
+ Hits        48121    48122    +1     
+ Misses      10158    10157    -1     

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