Update Windows base image to 1809 (#52)

Author: LazarusX

content/dotnet-template-azure-iot-edge-module/CSharp/Dockerfile.windows-amd64

@@ -7,7 +7,7 @@ RUN dotnet restore
 COPY . ./
 RUN dotnet publish -c Release -o out
 
-FROM microsoft/dotnet:2.1-runtime-nanoserver-1803
+FROM microsoft/dotnet:2.1-runtime-nanoserver-1809
 WORKDIR /app
 COPY --from=build-env /app/out ./
 ENTRYPOINT ["dotnet", "SampleModule.dll"]