chore(idtoken): fix some typos (#591)

idtoken/compute.go

diff --git a/idtoken/compute.go b/idtoken/compute.go
index 1b5e045..222396c 100644
--- a/idtoken/compute.go
+++ b/idtoken/compute.go
@@ -19,7 +19,7 @@
 // use the metadata service to build a TokenSource that fetches ID tokens.
 func computeTokenSource(audience string, ds *internal.DialSettings) (oauth2.TokenSource, error) {
 	if ds.CustomClaims != nil {
-		return nil, fmt.Errorf("idtoken: WithCustomClaims can't be used with the metadata serive, please provide a service account if you would like to use this feature")
+		return nil, fmt.Errorf("idtoken: WithCustomClaims can't be used with the metadata service, please provide a service account if you would like to use this feature")
 	}
 	ts := computeIDTokenSource{
 		audience: audience,

idtoken/doc.go

diff --git a/idtoken/doc.go b/idtoken/doc.go
index 6181aaf..1b1effd 100644
--- a/idtoken/doc.go
+++ b/idtoken/doc.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package idtoken provides utilities for creating authenticated transorts with
+// Package idtoken provides utilities for creating authenticated transports with
 // ID Tokens for Google HTTP APIs. It also provides methods to validate Google
 // issued ID tokens.
 package idtoken