os: fix documentation typos: s/an array/a slice/.

R=dsymonds
CC=golang-dev
https://golang.org/cl/6736057

src/pkg/os/doc.go

@@ -89,7 +89,7 @@ func Hostname() (name string, err error) {
 }
 
 // Readdir reads the contents of the directory associated with file and
-// returns an array of up to n FileInfo values, as would be returned
+// returns a slice of up to n FileInfo values, as would be returned
 // by Lstat, in directory order. Subsequent calls on the same file will yield
 // further FileInfos.
 //

src/pkg/os/file.go

@@ -185,7 +185,7 @@ func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
 }
 
 // WriteString is like Write, but writes the contents of string s rather than
-// an array of bytes.
+// a slice of bytes.
 func (f *File) WriteString(s string) (ret int, err error) {
 	if f == nil {
 		return 0, ErrInvalid