ipn/ipnlocal: fix doctor API endpoint (#11155)

Small fix to make sure doctor API endpoint returns correctly - I spotted it when checking my tailscaled node and noticed it was handled slightly different compare to the rest

Signed-off-by: San <santrancisco@users.noreply.github.com>

ipn/ipnlocal/peerapi.go

@@ -351,6 +351,7 @@ func (h *peerAPIHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	case "/v0/doctor":
 		h.handleServeDoctor(w, r)
+		return
 	case "/v0/sockstats":
 		h.handleServeSockStats(w, r)
 		return