Fixed multiseries prediction interval labels

[christopherbunn]

Resolves #4378

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8349680) 99.7% compared to head (b2145fa) 99.7%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4377 +/- ## ======================================= + Coverage 99.7% 99.7% +0.1% ======================================= Files 357 357 Lines 39922 39928 +6 ======================================= + Hits 39796 39802 +6 Misses 126 126 ```

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