Update CONTRIBUTING.md

[waitingkuo]

expression code moved to datafusion-expr #2669

Which issue does this PR close?

No issue, just update doc

Rationale for this change

What changes are included in this PR?

doc

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #2876 (0bef54f) into master (a1a6976) will not change coverage.
The diff coverage is n/a.

❗ Current head 0bef54f differs from pull request most recent head 8eaecc2. Consider uploading reports for the commit 8eaecc2 to get more accurate results

@@           Coverage Diff           @@
##           master    #2876   +/-   ##
=======================================
  Coverage   85.24%   85.24%           
=======================================
  Files         275      275           
  Lines       49010    49010           
=======================================
  Hits        41781    41781           
  Misses       7229     7229           

Impacted Files	Coverage Δ
datafusion/expr/src/window_frame.rs	92.43% <0.00%> (-0.85%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	74.16% <0.00%> (-0.20%)	⬇️
datafusion/core/src/physical_plan/metrics/value.rs	87.43% <0.00%> (+1.00%)	⬆️

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[alamb]

Thank you @waitingkuo