dt-bindings: net: renesas,ether: Update Sergei's email address

Update Sergei's email address, as per commit 534a8bf0cc
("MAINTAINERS: switch to my private email for Renesas Ethernet
drivers").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Sergei Shtylyov <sergei.shtylyov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/renesas,ether.yaml

@@ -10,7 +10,7 @@ allOf:
   - $ref: ethernet-controller.yaml#
 
 maintainers:
-  - Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
+  - Sergei Shtylyov <sergei.shtylyov@gmail.com>
 
 properties:
   compatible: