Fix uppercase G in GC mark assist goroutine IDs

[tbg]

Summary

• PR schedstat: add comprehensive GC analysis from range events #5 (GC analysis) introduced G%-8d in the mark assist per-goroutine output, missing the lowercase g convention established in PR Use lowercase g prefix for goroutine IDs #6.
• Fixes the one remaining uppercase G prefix and rewrites test expectations.

Test plan

• go test ./... -rewrite passes with updated testdata.