go.tools/go/types: fix typo in comment

TBR=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/135960043

go/types/api.go

@@ -121,7 +121,7 @@ type Config struct {
 	Import Importer
 
 	// If Sizes != nil, it provides the sizing functions for package unsafe.
-	// Otherwise &StdSize{WordSize: 8, MaxAlign: 8} is used instead.
+	// Otherwise &StdSizes{WordSize: 8, MaxAlign: 8} is used instead.
 	Sizes Sizes
 }