update feature gates doc (#4755)

Author: fatedier

README.md

@@ -1283,9 +1283,7 @@ frp supports feature gates to enable or disable experimental features. This allo
 To enable an experimental feature, add the feature gate to your configuration:
 
 ```toml
-featureGates = {
-  VirtualNet = true
-}
+featureGates = { VirtualNet = true }
 ```
 
 ### Feature Lifecycle

doc/virtual_net.md

@@ -49,9 +49,7 @@ type = "virtual_net"
 # frpc.toml (client side)
 serverAddr = "x.x.x.x"
 serverPort = 7000
-featureGates = {
-  VirtualNet = true
-}
+featureGates = { VirtualNet = true }
 
 # Configure the virtual network interface
 virtualNet.address = "100.86.0.2/24"