regenerate plugin proto with correct tooling

The proto file is structurally unchanged, though it now requires a go
import path option.

docs/plugin-protocol/tfplugin5.2.proto

@@ -17,6 +17,7 @@
 // branch or any other development branch.
 //
 syntax = "proto3";
+option go_package = "github.com/hashicorp/terraform/internal/tfplugin5";
 
 package tfplugin5;