[vpython] Fix PEP425 naming.

vpython/api/vpython/pep425.proto

Index: vpython/api/vpython/pep425.proto
diff --git a/vpython/api/vpython/pep425.proto b/vpython/api/vpython/pep425.proto
index 803b4565e45522ac3c4f4d9349fa017fb6c04477..c721a80375a218322ca51da37abdd8726d018f90 100644
--- a/vpython/api/vpython/pep425.proto
+++ b/vpython/api/vpython/pep425.proto
@@ -6,13 +6,13 @@ syntax = "proto3";
 
 package vpython;
 
-// Represnets a Python PEP425 tag.
-message Pep425Tag {
-  // Version is the PEP425 version tag (e.g., "cp27").
-  string version  = 1;
-  // ABI is the PEP425 ABI tag (e.g., "cp27mu", "none").
+// Represents a Python PEP425 tag.
+message PEP425Tag {
+  // Python is the PEP425 python tag (e.g., "cp27").
+  string python  = 1;
+  // ABI is the PEP425 "python ABI" tag (e.g., "cp27mu", "none").
   string abi = 2;
-  // Arch is the PEP425 architecture tag (e.g., "linux_x86_64", "armv7l",
-  // "any").
-  string arch = 3;
+  // Platform is the PEP425 "python platform" tag (e.g., "linux_x86_64",
+  // "armv7l", "any").
+  string platform = 3;
 }