[DOC] update mxnet version in installation doc

[eric-haibin-lin]

Description

The doc is wrong. Updated to 1.6.

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

[codecov]

Codecov Report

Merging #1072 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1072      +/-   ##
==========================================
+ Coverage    88.3%   88.32%   +0.02%     
==========================================
  Files          67       67              
  Lines        6266     6263       -3     
==========================================
- Hits         5533     5532       -1     
+ Misses        733      731       -2

Impacted Files	Coverage Δ
src/gluonnlp/utils/parameter.py	83.92% <ø> (+2.57%)	⬆️

[mli]

Job PR-1072/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1072/2/index.html

[leezu]

ERROR: No matching distribution found for mxnet>=1.6.0

@szha let's update this with the html side for pre-releases once it's ready.

[szha]

I think this only applies to the doc for master branch, which is updated automatically.

[leezu]

@szha yes, but it's currently broken even for the master branch. So the command should be fixed once the html page listing mxnet pre-releases for pip is ready.

[eric-haibin-lin]

That's reasonable. i bought up that we should publish the nightly build links to mxnet website on the dev list. But they are not yet available there. Then we can add a link on the gluonnlp master branch to all nightly builds.