crypto/x509: improve internal comment

Apply code review suggestion from CL 46715.

The block is doing more than just checking len(r.certs) == len(tc.cns).
It also verifies that certificate common names match.

Change-Id: I28d6926a5da48bd8f6c80aa5e5a1ed6d4990f845
Reviewed-on: https://go-review.googlesource.com/47132Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/root_unix_test.go

@@ -110,7 +110,7 @@ func TestEnvVars(t *testing.T) {
 				t.Fatal("nil roots")
 			}
 
-			// Verify len(r.certs) == len(tc.cns), otherwise report where the mismatch is.
+			// Verify that the returned certs match, otherwise report where the mismatch is.
 			for i, cn := range tc.cns {
 				if i >= len(r.certs) {
 					t.Errorf("missing cert %v @ %v", cn, i)