docs(typo): Specify valid values for select#nzNotFoundContent

[CygnusRoboticus]

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/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?
Docs update

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

What is the current behavior?

Incorrect valid values specified

Issue Number: N/A

What is the new behavior?

Corrected

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

N/A

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[codecov]

Codecov Report

Merging #3577 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #3577      +/-   ##
=========================================
- Coverage    95.7%   95.7%   -0.01%     
=========================================
  Files         690     690              
  Lines       13699   13699              
  Branches     1823    1823              
=========================================
- Hits        13111   13110       -1     
  Misses        200     200              
- Partials      388     389       +1

Impacted Files	Coverage Δ
components/time-picker/time-holder.ts	96.57% <0%> (-0.69%)	⬇️

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[vthinkxie]

LGTM