archive/tar: fix typo in comment

Change-Id: Ifcc565b34b3c3bb7ee62bb0525648a5d2895bf0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/282013
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Alberto Donizetti <alb.donizetti@gmail.com>

src/archive/tar/strconv.go

@@ -28,7 +28,7 @@ func isASCII(s string) bool {
 }
 
 // toASCII converts the input to an ASCII C-style string.
-// This a best effort conversion, so invalid characters are dropped.
+// This is a best effort conversion, so invalid characters are dropped.
 func toASCII(s string) string {
 	if isASCII(s) {
 		return s