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

R=dsymonds
CC=golang-dev
https://golang.org/cl/6736057
diff --git a/src/pkg/os/file.go b/src/pkg/os/file.go
index 4acf35d..5b1658a 100644
--- a/src/pkg/os/file.go
+++ b/src/pkg/os/file.go
@@ -185,7 +185,7 @@
 }
 
 // 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