fix: addExternalSubnets -> extraExternalSubnets (#230)

Signed-off-by: zcq98 <[email protected]>

Author: zhaocongqi

docs/advance/ovn-eip-fip-snat.en.md

@@ -256,7 +256,7 @@ spec:
   namespaces:
   - vpc1
   enableExternal: true  # vpc enableExternal will automatically create an lrp association to the public network specified above
-  addExternalSubnets: # configure addExternalSubnets to support connecting multiple additional public networks
+  extraExternalSubnets: # configure extraExternalSubnets to support connecting multiple additional public networks
   - extra
 ```
 

docs/advance/ovn-eip-fip-snat.md

@@ -254,7 +254,7 @@ spec:
   namespaces:
   - vpc1
   enableExternal: true  # 开启 enableExternal 后 vpc 会自动连接名为 external 的 ls
-  addExternalSubnets: # 配置 addExternalSubnets 支持连接多个额外的公网网络
+  extraExternalSubnets: # 配置 extraExternalSubnets 支持连接多个额外的公网网络
   - extra
 ```