Fix array shape

[hrzn]

This fixes an error that appeared with latest version of sklearn.

[codecov-commenter]

Codecov Report

Merging #963 (a66c526) into master (5213854) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #963   +/-   ##
=======================================
  Coverage   92.60%   92.60%           
=======================================
  Files          74       74           
  Lines        7398     7399    +1     
=======================================
+ Hits         6851     6852    +1     
  Misses        547      547           

Impacted Files	Coverage Δ
darts/models/filtering/gaussian_process_filter.py	100.00% <100.00%> (ø)

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