fix readme instructions to reflect new structure

[marcvanheerden]

Which issue does this PR close?

Closes #523 .

Rationale for this change

The readme was out of date

What changes are included in this PR?

update code snippets in readme developer instructions section

Are there any user-facing changes?

No

[alamb]

Thank you @marcvanheerden !

[codecov-commenter]

Codecov Report

Merging #524 (42668a7) into master (21d69ca) will not change coverage.
The diff coverage is n/a.

❗ Current head 42668a7 differs from pull request most recent head 3015f55. Consider uploading reports for the commit 3015f55 to get more accurate results

@@           Coverage Diff           @@
##           master     #524   +/-   ##
=======================================
  Coverage   82.54%   82.54%           
=======================================
  Files         167      167           
  Lines       45956    45956           
=======================================
  Hits        37933    37933           
  Misses       8023     8023           

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[alamb]

https://github.com/apache/arrow-rs/pull/524/checks?check_run_id=3001616900 is unrelated to this PR (I think @ritchie46 fixed it in #525); Merging in