docs(module: website): del undefined msg in website

[fisherspy]

PR Checklist

Please check if your PR fulfills the following requirements:

• [ ] The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd-mobile/blob/master/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[x] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov[bot]]

Codecov Report

Merging #727 into master will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #727      +/-   ##
==========================================
+ Coverage   87.14%   87.16%   +0.01%     
==========================================
  Files         164      164              
  Lines        6747     6747              
  Branches     1104     1104              
==========================================
+ Hits         5880     5881       +1     
  Misses        421      421              
+ Partials      446      445       -1     

Impacted Files	Coverage Δ
...ts/input-item/custom-input/custom-input.service.ts	87.09% <0.00%> (+3.22%)	:arrow_up:

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