various: adds missing apostrophes to comments

Updates #cleanup

Change-Id: I7bf29cc153c3c04e087f9bdb146c3437bed0129a
Signed-off-by: Alex Chan <alexc@tailscale.com>

tka/key_test.go

@@ -42,7 +42,7 @@ func TestVerify25519(t *testing.T) {
 	aum := AUM{
 		MessageKind: AUMRemoveKey,
 		KeyID:       []byte{1, 2, 3, 4},
-		// Signatures is set to crap so we are sure its ignored in the sigHash computation.
+		// Signatures is set to crap so we are sure it's ignored in the sigHash computation.
 		Signatures: []tkatype.Signature{{KeyID: []byte{45, 42}}},
 	}
 	sigHash := aum.SigHash()
@@ -89,7 +89,7 @@ func TestNLPrivate(t *testing.T) {
 		t.Error("signature did not verify")
 	}
 
-	// We manually compute the keyID, so make sure its consistent with
+	// We manually compute the keyID, so make sure it's consistent with
 	// tka.Key.ID().
 	if !bytes.Equal(k.MustID(), p.KeyID()) {
 		t.Errorf("private.KeyID() & tka KeyID differ: %x != %x", k.MustID(), p.KeyID())