runtime/trace: fix a typo in doc

Change-Id: I63f3d2edb09801c99957a1f744639523fb6d0b62
Reviewed-on: https://go-review.googlesource.com/60331Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/trace/trace.go

@@ -7,7 +7,7 @@
 //
 // The execution trace captures a wide range of execution events such as
 // goroutine creation/blocking/unblocking, syscall enter/exit/block,
-// GC-related events, changes of heap size, processor stop/stop, etc.
+// GC-related events, changes of heap size, processor start/stop, etc.
 // A precise nanosecond-precision timestamp and a stack trace is
 // captured for most events. The generated trace can be interpreted
 // using `go tool trace`.