add getSubnet to p-chain api reference

[felipemadero]

Why this should be merged

Doc is missing and the API call is operative

How this works

Fix the doc

How this was tested

[StephenButtolph]

I don't think id exists in the return type:

avalanchego/vms/platformvm/service.go

Lines 430 to 440 in f4d8a3c

	// GetSubnetResponse is the response from calling GetSubnet
	type GetSubnetResponse struct {
	// whether it is permissioned or not
	IsPermissioned bool `json:"isPermissioned"`
	// subnet auth information for a permissioned subnet
	ControlKeys []string `json:"controlKeys"`
	Threshold avajson.Uint32 `json:"threshold"`
	Locktime avajson.Uint64 `json:"locktime"`
	// subnet transformation tx ID for a permissionless subnet
	SubnetTransformationTxID ids.ID `json:"subnetTransformationTxID"`
	}