[Serverless Docs] fixing a wrong call out to "search boost window"

[Kushmaro]

In this docs page we refer to "Time series data retention" however, the correct concept to depict is "Search boost window" i.e line should read:

"Search boost window: By limiting the number of days of time series data that are available for caching, you can reduce the number of search VCUs required."

Ive opened this issue as a followup to this closed PR due to the docs freeze.