os: add missing full stop in comment

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6399047

src/pkg/os/types.go

@@ -12,7 +12,7 @@ import (
 // Getpagesize returns the underlying system's memory page size.
 func Getpagesize() int { return syscall.Getpagesize() }
 
-// A FileInfo describes a file and is returned by Stat and Lstat
+// A FileInfo describes a file and is returned by Stat and Lstat.
 type FileInfo interface {
 	Name() string       // base name of the file
 	Size() int64        // length in bytes for regular files; system-dependent for others