fix(module:pulltorefresh): fix element height

[nuonuoge]

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)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] 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]

Codecov Report

Merging #699 into 1.x will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              1.x     #699   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files         270      270           
  Lines        7622     7622           
  Branches     1097     1097           
=======================================
  Hits         6753     6753           
  Misses        422      422           
  Partials      447      447           

Impacted Files	Coverage Δ
...nents/pull-to-refresh/pull-to-refresh.component.ts	74.35% <100.00%> (ø)

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