syscall: fix typo in comment

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/120570043

src/pkg/syscall/syscall_unix.go

@@ -56,7 +56,7 @@ func (m *mmapper) Mmap(fd int, offset int64, length int, prot int, flags int) (d
 		cap  int
 	}{addr, length, length}
 
-	// Use unsafeto turn sl into a []byte.
+	// Use unsafe to turn sl into a []byte.
 	b := *(*[]byte)(unsafe.Pointer(&sl))
 
 	// Register mapping in m and return it.