runtime: update go doc on runtime.gcMarkRootPrepare

[Petelin]

// gcMarkRootPrepare queues root scanning jobs (stacks, globals, and
// some miscellany) and initializes scanning-related state.
//
// The caller must have call gcCopySpans().
//
// The world must be stopped.
//
//go:nowritebarrier
func gcMarkRootPrepare() {

this gcCopySpans() already not exist.

[ianlancetaylor]

CC @aclements

[gopherbot]

Change https://golang.org/cl/209917 mentions this issue: runtime: remove comment about gcCopySpans()