Fix indices for prediction intervals

[eccabay]

Closes #4153

[codecov]

Codecov Report

Merging #4154 (6866c9c) into main (21eb2ff) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #4154     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        349     349             
  Lines      37778   37780      +2     
=======================================
+ Hits       37661   37663      +2     
  Misses       117     117             

Impacted Files	Coverage Δ
...valml/pipelines/time_series_regression_pipeline.py	100.0% <100.0%> (ø)

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