dt-bindings: net: fec: Fix indentation

The following warning is observed when running 'make dtbs_check':
Documentation/devicetree/bindings/net/fsl,fec.yaml:85:7: [warning] wrong indentation: expected 8 but found 6 (indentation)

Fix the indentation accordingly.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/fsl,fec.yaml

@@ -82,11 +82,11 @@ properties:
     maxItems: 5
     contains:
       enum:
-      - ipg
-      - ahb
-      - ptp
-      - enet_clk_ref
-      - enet_out
+        - ipg
+        - ahb
+        - ptp
+        - enet_clk_ref
+        - enet_out
 
   phy-mode: true