runtime/trace: fix file name in example

The preceding paragraph suggests the test run will produce a file called trace.out.
The same name, trace.out, is used in the output from go help testflag, thus we change the go test line instead of changing the preceding paragraph.

Change-Id: Ib1fa7e49e540853e263a2399b16040ea6f41b703
GitHub-Last-Rev: 3535e62bf8dd02dd9955a28e7fa9cca98de89efd
GitHub-Pull-Request: golang/go#39709
Reviewed-on: https://go-review.googlesource.com/c/go/+/238997


Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>