fix(mqtt): unbreak the .Contracts csproj -- '--' is illegal in an XML comment

The build-platform note added alongside the proto wiring quoted docker-dev's
`FROM --platform=linux/amd64` verbatim inside an MSBuild comment. XML forbids
'--' in a comment, so MSBuild refused to load the project at all (MSB4025) --
a total build stop for every consumer of .Contracts, i.e. the whole MQTT
driver, the Host, and the test suite. Reworded to name the platform in prose.

Caught by rebuilding after the edit; the preceding commit was made from a
build that predated it.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Contracts.csproj

@@ -20,8 +20,8 @@
 
          Build-platform note (inherited, not new): Grpc.Tools' bundled linux_arm64 protoc segfaults
          (exit 139) under Apple-Silicon Docker, which is why docker-dev/Dockerfile pins its build stage
-         to --platform=linux/amd64. That pin already exists for the Commons proto; this project adds a
-         second .proto to the same already-pinned build, not a new constraint. -->
+         to the linux/amd64 platform. That pin already exists for the Commons proto; this project adds
+         a second .proto to the same already-pinned build, not a new constraint. -->
     <ItemGroup>
         <PackageReference Include="Google.Protobuf"/>
         <PackageReference Include="Grpc.Tools">