os: fix typo in comment

fix misspell on code comment.

Change-Id: I8280488f71f55616da5543eac0f58cc008354af9
GitHub-Last-Rev: ea144718ee5f6dccb3b694100e9edda923e576fc
GitHub-Pull-Request: golang/go#24975
Reviewed-on: https://go-review.googlesource.com/108458Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>

src/os/os_windows_test.go

@@ -191,7 +191,7 @@ func (rd *reparseData) addUTF16s(s []uint16) (offset uint16) {
 
 func (rd *reparseData) addString(s string) (offset, length uint16) {
 	p := syscall.StringToUTF16(s)
-	return rd.addUTF16s(p), uint16(len(p)-1) * 2 // do not include terminating NUL in the legth (as per PrintNameLength and SubstituteNameLength documentation)
+	return rd.addUTF16s(p), uint16(len(p)-1) * 2 // do not include terminating NUL in the length (as per PrintNameLength and SubstituteNameLength documentation)
 }
 
 func (rd *reparseData) addSubstituteName(name string) {