Fix MinMaxDifferenceTransform import

[Mr-Geekman]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

Fix import.

Closing issues

[github-actions]

🚀 Deployed on https://deploy-preview-1030--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #1030 (7f7d6a4) into master (04570b2) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1030      +/-   ##
==========================================
+ Coverage   86.10%   86.16%   +0.05%     
==========================================
  Files         162      162              
  Lines        8622     8628       +6     
==========================================
+ Hits         7424     7434      +10     
+ Misses       1198     1194       -4     

Impacted Files	Coverage Δ
etna/transforms/__init__.py	100.00% <100.00%> (ø)
etna/models/utils.py	100.00% <0.00%> (ø)
etna/models/nn/mlp.py	100.00% <0.00%> (ø)
etna/models/nn/rnn.py	100.00% <0.00%> (ø)
etna/models/nn/tft.py	99.11% <0.00%> (+<0.01%)	⬆️
etna/models/nn/deepar.py	98.94% <0.00%> (+0.01%)	⬆️
etna/auto/utils.py	94.73% <0.00%> (+21.05%)	⬆️

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[martins0n]

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