Update ballista readme link

[tustvold]

Which issue does this PR close?

Closes #.

Rationale for this change

Ballista has moved, update link

What changes are included in this PR?

Updates Ballista README.md link

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #1765 (adb3711) into master (8e1666a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1765   +/-   ##
=======================================
  Coverage   83.47%   83.48%           
=======================================
  Files         196      196           
  Lines       55923    55949   +26     
=======================================
+ Hits        46684    46710   +26     
  Misses       9239     9239           

Impacted Files	Coverage Δ
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.23%)	⬇️
arrow/src/compute/kernels/cast.rs	95.77% <0.00%> (-0.05%)	⬇️
parquet/src/encodings/encoding.rs	93.65% <0.00%> (+0.19%)	⬆️
arrow/src/array/transform/mod.rs	86.96% <0.00%> (+0.22%)	⬆️

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[HaoYang670]

LGTM!