[vpython] Fix PEP425 naming.

vpython/api/vpython/env.proto

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 syntax = "proto3";
 
 package vpython;
 
 import "github.com/luci/luci-go/vpython/api/vpython/pep425.proto";
 import "github.com/luci/luci-go/vpython/api/vpython/spec.proto";
 
 
 // Environment describes a constructed VirtualEnv.
 message Environment {
   // A constructed VirtualEnv.
   vpython.Spec spec = 1;
 
   // The resolved runtime parameters.
   Runtime runtime = 2;
 
   // The PEP425 tags that were probed for this Python environment.
-  repeated vpython.Pep425Tag pep425_tag = 3;
+  repeated vpython.PEP425Tag pep425_tag = 3;
 }
 
 
 // Runtime is the set of resolved runtime parameters.
 message Runtime {
   // The absolute path to the resolved interpreter.
   string path = 1;
   // The SHA256 hash of the resolved interpreter.
   string hash = 2;
   // The resolved Python interpreter version.
   string version = 3;
 }