Avoiding string to bytes conversion example is wrong.

[cuonglm]

At b85fa2d, in this file, both BenchmarkBad and BenchmarkGood causes []byte("Hello world") escape to heap:

 $ go test -c -gcflags=-m p_test.go 
# command-line-arguments [command-line-arguments.test]
./p_test.go:8:19: b does not escape
./p_test.go:11:14: ([]byte)("Hello world") escapes to heap
./p_test.go:14:13: ... argument does not escape
./p_test.go:14:14: a escapes to heap
./p_test.go:17:20: b does not escape
./p_test.go:19:17: ([]byte)("Hello world") escapes to heap
./p_test.go:23:13: ... argument does not escape
./p_test.go:23:14: a escapes to heap
# command-line-arguments.test
_testmain.go:39:6: can inline init.0
_testmain.go:47:24: inlining call to testing.MainStart
_testmain.go:47:42: testdeps.TestDeps{} escapes to heap
_testmain.go:47:24: &testing.M{...} escapes to heap

The reason BenchmarkBad is worse because the escaping happens inside the loop, it does not relate to string to bytes conversion operation.