Revert "cmd/compilebench: update example to use benchcmp instead of benchstat"

This reverts commit 3aa1caa08a.

Reason for revert: benchstat is preferred to benchcmp

Change-Id: I9772eb342fb631a68511986d832bb1444766afdc
Reviewed-on: https://go-review.googlesource.com/42027
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/compilebench/main.go

@@ -61,7 +61,7 @@
 //
 //	compilebench -count 10 -compile $(toolstash -n compile) >old.txt
 //	compilebench -count 10 >new.txt
-//	benchcmp old.txt new.txt
+//	benchstat old.txt new.txt
 //
 package main