api: update keyring comment to reflect correct feature name. (#14558)

api/keyring.go

@@ -5,7 +5,7 @@ import (
 	"net/url"
 )
 
-// Keyring is used to access the Secure Variables keyring
+// Keyring is used to access the Variables keyring.
 type Keyring struct {
 	client *Client
 }