minor typo fix (#17)

diff --git a/subcommands.go b/subcommands.go
index 951a4f6..9cb98e5 100644
--- a/subcommands.go
+++ b/subcommands.go
@@ -360,7 +360,7 @@
 }
 
 // An aliaser is a Command wrapping another Command but returning a
-// different name as it's alias
+// different name as its alias.
 type aliaser struct {
 	alias string
 	Command