add module exports for custom schedule in document

[samwu4166]

current document will cause schedule error because scheduler can't find schedule property, so just added module.exports would work.

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

[codecov]

Codecov Report

Merging #52 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #52   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          191       191           
=========================================
  Hits           191       191           

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