os: fix minor typo
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5697051
diff --git a/src/pkg/os/file.go b/src/pkg/os/file.go
index 1c3d017..4acf35d 100644
--- a/src/pkg/os/file.go
+++ b/src/pkg/os/file.go
@@ -25,7 +25,7 @@
// open file.go: no such file or directory
//
// The file's data can then be read into a slice of bytes. Read and
-// Write take their byte counts from the length of the artument slice.
+// Write take their byte counts from the length of the argument slice.
//
// data := make([]byte, 100)
// count, err := file.Read(data)
