MAINTAINERS: fix area Networking issue assignment

Replacing "area: Networking" with "area: PTP" to prevent from
including myself in issues and PRs that are related to every
piece of Networking code. Insted I'll be included only in PTP
related issued/PRs.

Signed-off-by: Adam Wojasinski <[email protected]>

Author: awojasinski

MAINTAINERS.yml

@@ -2998,7 +2998,7 @@ Networking:
     - subsys/net/lib/ptp/
     - samples/net/ptp/
   labels:
-    - "area: Networking"
+    - "area: PTP"
   tests:
     - sample.net.ptp