[DOC] update BERT model zoo page

[eric-haibin-lin]

Description

As title.

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request head (zoo@e0b2173). Click here to learn what that means.
The diff coverage is n/a.

[codecov]

Codecov Report

Merging #762 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #762     +/-   ##
=========================================
- Coverage   90.79%   90.69%   -0.1%     
=========================================
  Files          64       64             
  Lines        6113     6113             
=========================================
- Hits         5550     5544      -6     
- Misses        563      569      +6

Impacted Files	Coverage Δ
src/gluonnlp/data/dataloader.py	83.62% <0%> (-5.18%)	⬇️

[mli]

Job PR-762/1 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-762/1/index.html