tailcfg: fix broken test from comment change

Fix broken build from 255c0472fb

"Oh, that's safe to commit because most tests are passing and it's
just a comment change!", I thought, forgetting I'd added a test that
parses its comments.

Change-Id: Iae93d595e06fec48831215a98adbb270f3bfda05
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg_test.go

@@ -661,7 +661,7 @@ func TestRegisterRequestNilClone(t *testing.T) {
 // We've screwed this up several times.
 func TestCurrentCapabilityVersion(t *testing.T) {
 	f := must.Get(os.ReadFile("tailcfg.go"))
-	matches := regexp.MustCompile(`(?m)^//\s+(\d+): \d\d\d\d-\d\d-\d\d: `).FindAllStringSubmatch(string(f), -1)
+	matches := regexp.MustCompile(`(?m)^//[\s-]+(\d+): \d\d\d\d-\d\d-\d\d: `).FindAllStringSubmatch(string(f), -1)
 	max := 0
 	for _, m := range matches {
 		n := must.Get(strconv.Atoi(m[1]))