Minor update topByKey comments

[liufuyang]

Make comment matching function definiation.

[codecov]

Codecov Report

Merging #2695 into master will decrease coverage by 1.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2695      +/-   ##
==========================================
- Coverage    71.9%   70.45%   -1.46%     
==========================================
  Files         208      208              
  Lines        7308     7308              
  Branches      341      350       +9     
==========================================
- Hits         5255     5149     -106     
- Misses       2053     2159     +106

Impacted Files	Coverage Δ
...spotify/scio/values/PairSCollectionFunctions.scala	96.89% <ø> (ø)	⬆️
...scala/com/spotify/scio/bigquery/client/Cache.scala	0% <0%> (-66.67%)	⬇️
.../spotify/scio/bigquery/BigQueryPartitionUtil.scala	0% <0%> (-52.5%)	⬇️
.../spotify/scio/bigquery/client/BigQueryConfig.scala	0% <0%> (-41.18%)	⬇️
...otify/scio/bigquery/syntax/ScioContextSyntax.scala	16.66% <0%> (-33.34%)	⬇️
.../scala/com/spotify/scio/bigquery/StorageUtil.scala	0% <0%> (-31.71%)	⬇️
...com/spotify/scio/bigquery/types/BigQueryType.scala	58.33% <0%> (-29.17%)	⬇️
...com/spotify/scio/bigquery/types/TypeProvider.scala	50.72% <0%> (-23.19%)	⬇️
...n/scala/com/spotify/scio/bigquery/BigQueryIO.scala	37.98% <0%> (-3.85%)	⬇️
...y/scio/values/PairSkewedSCollectionFunctions.scala	90.62% <0%> (-3.13%)	⬇️
... and 4 more

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[regadas]

Thanks!