ARROW-2978: [Rust] Change argument to rust fmt to fix build

[andygrove]

Work in progress... trying to fix the build.

[andygrove]

@wesm This fixes the Rust part of the build ... at time of writing I am waiting for the JDK build to finish ... but once that is done this good to merge.

[codecov-io]

Codecov Report

Merging #2371 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2371      +/-   ##
==========================================
+ Coverage   84.43%   84.49%   +0.05%     
==========================================
  Files         281      293      +12     
  Lines       44418    45329     +911     
==========================================
+ Hits        37505    38301     +796     
- Misses       6882     6997     +115     
  Partials       31       31

Impacted Files	Coverage Δ
rust/src/array.rs	85.58% <100%> (ø)
rust/src/datatypes.rs	71.96% <100%> (ø)
rust/src/buffer.rs	89.13% <100%> (ø)
cpp/src/arrow/util/thread-pool-test.cc	98.91% <0%> (-0.55%)	⬇️
rust/src/lib.rs	100% <0%> (ø)
rust/src/error.rs	0% <0%> (ø)
rust/src/builder.rs	97.81% <0%> (ø)
rust/src/record_batch.rs	100% <0%> (ø)
rust/src/list.rs	100% <0%> (ø)
rust/src/bitmap.rs	100% <0%> (ø)
... and 6 more

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[wesm]

+1. Thanks @andygrove!