[DOC] fix broken links

[szha]

Description

fix broken links

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

• fix broken links

[codecov]

Codecov Report

Merging #833 into master will increase coverage by 9.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #833      +/-   ##
==========================================
+ Coverage   81.02%   90.18%   +9.15%     
==========================================
  Files          66       66              
  Lines        6345     6345              
==========================================
+ Hits         5141     5722     +581     
+ Misses       1204      623     -581

Impacted Files	Coverage Δ
src/gluonnlp/data/corpora/wikitext.py	100% <ø> (ø)	⬆️
src/gluonnlp/data/glue.py	98.63% <0%> (+1.81%)	⬆️
src/gluonnlp/model/train/embedding.py	87.26% <0%> (+2.54%)	⬆️
src/gluonnlp/embedding/token_embedding.py	92.01% <0%> (+2.83%)	⬆️
src/gluonnlp/vocab/elmo.py	96.66% <0%> (+3.33%)	⬆️
src/gluonnlp/data/word_embedding_evaluation.py	96.96% <0%> (+3.4%)	⬆️
src/gluonnlp/data/registry.py	78.12% <0%> (+6.25%)	⬆️
src/gluonnlp/data/stream.py	85.05% <0%> (+7.21%)	⬆️
src/gluonnlp/model/utils.py	77.58% <0%> (+7.75%)	⬆️
src/gluonnlp/model/transformer.py	90.05% <0%> (+9.16%)	⬆️
... and 11 more

[mli]

Job PR-833/12 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-833/12/index.html