Fix typo in data_pipeline

[akihironitta]

What does this PR do?

Fix a small typo in DataPipeline.

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• [n/a] Did you write any new necessary tests? [not needed for typos/docs]
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[codecov]

Codecov Report

Merging #220 (94093b1) into master (14502b0) will decrease coverage by 3.74%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
- Coverage   86.56%   82.81%   -3.75%     
==========================================
  Files          57       57              
  Lines        2747     2747              
==========================================
- Hits         2378     2275     -103     
- Misses        369      472     +103     

Flag	Coverage Δ
unittests	82.81% <100.00%> (-3.75%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/data/data_pipeline.py	88.04% <100.00%> (ø)
flash/text/seq2seq/core/data.py	42.62% <0.00%> (-40.99%)	⬇️
flash/text/classification/data.py	40.90% <0.00%> (-40.91%)	⬇️
flash/text/classification/model.py	62.50% <0.00%> (-31.25%)	⬇️
flash/text/seq2seq/summarization/data.py	57.14% <0.00%> (-28.58%)	⬇️
flash/text/seq2seq/translation/model.py	55.55% <0.00%> (-16.67%)	⬇️
flash/text/seq2seq/summarization/metric.py	84.48% <0.00%> (-12.07%)	⬇️
flash/text/seq2seq/translation/data.py	80.00% <0.00%> (-10.00%)	⬇️
flash/text/seq2seq/core/model.py	61.53% <0.00%> (-6.16%)	⬇️

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