[BUGFIX] Change CONLL dataset source to HTTPS

[chenw23]

Description

#1235

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

cc @dmlc/gluon-nlp-team

[mli]

Job PR-1236/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1236/1/index.html

[codecov]

Codecov Report

Merging #1236 into master will increase coverage by 6.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1236      +/-   ##
==========================================
+ Coverage   81.24%   87.45%   +6.20%     
==========================================
  Files          81       81              
  Lines        7365     7365              
==========================================
+ Hits         5984     6441     +457     
+ Misses       1381      924     -457     

Impacted Files	Coverage Δ
src/gluonnlp/data/conll.py	100.00% <100.00%> (+62.91%)	⬆️
src/gluonnlp/data/batchify/batchify.py	95.27% <0.00%> (+0.78%)	⬆️
src/gluonnlp/data/transforms.py	83.85% <0.00%> (+1.69%)	⬆️
src/gluonnlp/base.py	89.65% <0.00%> (+3.44%)	⬆️
src/gluonnlp/data/corpora/wikitext.py	100.00% <0.00%> (+5.26%)	⬆️
src/gluonnlp/model/parameter.py	100.00% <0.00%> (+8.00%)	⬆️
src/gluonnlp/data/dataset.py	98.63% <0.00%> (+10.20%)	⬆️
src/gluonnlp/data/word_embedding_evaluation.py	89.27% <0.00%> (+16.47%)	⬆️
src/gluonnlp/data/bert/squad.py	29.54% <0.00%> (+19.31%)	⬆️
src/gluonnlp/calibration/collector.py	26.66% <0.00%> (+26.66%)	⬆️
... and 5 more