Correct the PTP page: no PTP on a 4B, but the CM4 has it

Reader sent Geerling's CM4 PTP write-up, which shows two CM4s syncing to 10-15ns.
Our headline overclaimed ('impossible on a Pi 4') and our mechanism was wrong (we
blamed the bcmgenet MAC for not exposing a PHC).

The real story is better: the CM4 (BCM54210PE) and the 4B (BCM54213PE) report the
IDENTICAL PHY ID 0x600d84a2, so the kernel cannot tell them apart by asking the
chip. bcm-phy-ptp.c disambiguates by MDIO bus address instead — addr 0 = CM4,
proceed; addr 1 = 4B, return NULL. Ours is addr 1.

It is not a kernel config gap and not our RT patch: CONFIG_BCM_NET_PHYPTP=y and
CONFIG_NETWORK_PHY_TIMESTAMPING=y are both set. The driver loads, looks at the
board, and declines.

Correction is visible on the page rather than quietly patched. Credit to Geerling.

docs-site/astro.config.mjs

@@ -40,7 +40,7 @@ export default defineConfig({
       description:
         "What the Raspberry Pi time-server guides get wrong, and the numbers to prove it. " +
         "GPS Stratum 1 on a Pi 4: PREEMPT_RT makes PPS jitter worse, the PPS interrupt " +
-        "cannot be pinned, PTP is impossible, and your dashboard is taxing your clock.",
+        "cannot be pinned, PTP needs a CM4 not a 4B, and your dashboard is taxing your clock.",
 
       // The mark IS the word "cuckoo" — a rebus. replacesTitle stops Starlight
       // rendering the title text beside it (which would read "…escapement The