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ARROW-10915: [Rust] README.md: set the Env vars as absolute dirs; several minor fixes.

In rust/README.md,  both ARROW_TEST_DATA and PARQUET_TEST_DATA are set as relative path. The problem is: we MAY have to reset them back-and-forth across top and subdirectories – that's annoying. So, the obvious solution is: set the Env vars as absolute dirs.

This PR also contains several minor fixes to spelling and format!

Closes #8924 from mqy/abs_dirs

Authored-by: mqy <meng.qingyou@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>