path/filepath: fix typo

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6117063

src/pkg/path/filepath/path_plan9.go

@@ -12,7 +12,7 @@ func IsAbs(path string) bool {
 }
 
 // VolumeName returns the leading volume name on Windows.
-// It returns "" elsewhere
+// It returns "" elsewhere.
 func VolumeName(path string) string {
 	return ""
 }