Update cppguide.html

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https://developers.google.com/protocol-buffers/docs/reference/cpp-generated#package

cppguide.html

@@ -688,7 +688,7 @@ using ::foo::Bar;
   <code>.proto</code> file. See
 
 
-  <a href="https://developers.google.com/protocol-buffers/docs/reference/cpp-generated#package">
+  <a href="https://protobuf.dev/reference/cpp/cpp-generated/#package">
   Protocol Buffer Packages</a>
   for details.</li>