fix(cost-centers): use third-person verb in data source descriptions

[vmvarela]

Summary

• Closes chore(cost-centers): fix data source descriptions to use third-person verb #51

Changes

Updated Description field in both cost-center data sources and their corresponding docs to follow the AGENTS.md convention (third-person singular verb for data sources).

File	Before	After
data_source_github_enterprise_cost_center.go	"Use this data source to retrieve..."	"Retrieves information about a specific GitHub enterprise cost center."
data_source_github_enterprise_cost_centers.go	"Use this data source to retrieve..."	"Retrieves a list of GitHub enterprise cost centers."
website/docs/d/enterprise_cost_center.html.markdown	"Use this data source to retrieve..."	"Retrieves information about a specific GitHub enterprise cost center."
website/docs/d/enterprise_cost_centers.html.markdown	"Use this data source to list..."	"Retrieves a list of GitHub enterprise cost centers."