docs: fix customize theme iconfont import error

[nuonuoge]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] 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
[x] Documentation content changes
[ ] 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 #728 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #728   +/-   ##
=======================================
  Coverage   87.16%   87.16%           
=======================================
  Files         164      164           
  Lines        6747     6747           
  Branches     1104     1104           
=======================================
  Hits         5881     5881           
  Misses        421      421           
  Partials      445      445           

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