cmd/racebuild: clarify LLVM revision in usage comment

LLVM uses Subversion for their primary repository, so one might expect
that the rev passed to racebuild would be a Subversion revision
number. It is actually a git commit hash.

Updates golang/go#24354.

Change-Id: I8528f4f80477e256d889e3bbc98de6309f449c1c
Reviewed-on: https://go-review.googlesource.com/112875
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: Bryan C. Mills

cmd/racebuild/racebuild.go

@@ -4,7 +4,7 @@
 
 // racebuild builds the race runtime (syso files) on all supported OSes using gomote.
 // Usage:
-//	$ racebuild -rev <llvm_revision> -goroot <path_to_go_repo>
+//	$ racebuild -rev <llvm_git_revision> -goroot <path_to_go_repo>
 package main
 
 import (