fix typo (#822)

fs.go

@@ -14,7 +14,7 @@ type (
 	}
 )
 
-// Dir returns a http.Filesystem that can be used by http.FileServer(). It is used interally
+// Dir returns a http.Filesystem that can be used by http.FileServer(). It is used internally
 // in router.Static().
 // if listDirectory == true, then it works the same as http.Dir() otherwise it returns
 // a filesystem that prevents http.FileServer() to list the directory files.