os: fix documentation for FileInfo.Name.

It's the base name, not the full name.
Fixes #2047.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4680041

src/pkg/os/types.go

@@ -27,7 +27,7 @@ type FileInfo struct {
 	Atime_ns        int64  // access time; nanoseconds since epoch.
 	Mtime_ns        int64  // modified time; nanoseconds since epoch.
 	Ctime_ns        int64  // status change time; nanoseconds since epoch.
-	Name            string // name of file as presented to Open.
+	Name            string // base name of the file name provided in Open, Stat, etc.
 	FollowedSymlink bool   // followed a symlink to get this information
 }