small typo fix in clientcredentials.go

Change-Id: If564daf4d164c27b2748b9c4c4ff274f3601184c
Reviewed-on: https://go-review.googlesource.com/15805
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/clientcredentials/clientcredentials.go b/clientcredentials/clientcredentials.go
index baebced..fa0770c 100644
--- a/clientcredentials/clientcredentials.go
+++ b/clientcredentials/clientcredentials.go
@@ -66,7 +66,7 @@
 	Scopes []string
 }
 
-// Token uses client credentials to retreive a token.
+// Token uses client credentials to retrieve a token.
 // The HTTP client to use is derived from the context.
 // If nil, http.DefaultClient is used.
 func (c *Config) Token(ctx context.Context) (*oauth2.Token, error) {