fix(perf): Fix benchmark

The reader was being read only once in the first iteration, and
afterwards it was always empty, thus `parseCodeowners` was parsing an
empty file.

These were the metrics before the change:

    $ go test -benchmem -bench=.
    goos: darwin
    goarch: arm64
    pkg: github.com/hairyhenderson/go-codeowners
    BenchmarkParseCodeowners-10      2677503               465.3 ns/op          4096 B/op          1 allocs/op
    PASS
    ok      github.com/hairyhenderson/go-codeowners 2.255s

After the change it's not showing more realistic data:

    $ go test -benchmem -bench=.
    goos: darwin
    goarch: arm64
    pkg: github.com/hairyhenderson/go-codeowners
    BenchmarkParseCodeowners-10        31054             38910 ns/op           61424 B/op        641 allocs/op
    PASS
    ok      github.com/hairyhenderson/go-codeowners 3.529s

codeowners_test.go

@@ -145,10 +145,12 @@ func TestParseCodeownersSections(t *testing.T) {
 }
 
 func BenchmarkParseCodeowners(b *testing.B) {
-	r := bytes.NewBufferString(sample)
 	var c []Codeowner
 
 	for n := 0; n < b.N; n++ {
+		b.StopTimer()
+		r := bytes.NewBufferString(sample)
+		b.StartTimer()
 		c, _ = parseCodeowners(r)
 	}