Update chat_sample.cpp (openvinotoolkit#770)

Minor fixes -- ensure using the device variable instead of hard coding
"CPU"

samples/cpp/chat_sample/chat_sample.cpp

@@ -11,7 +11,7 @@ int main(int argc, char* argv[]) try {
     std::string model_path = argv[1];
 
     std::string device = "CPU";  // GPU, NPU can be used as well
-    ov::genai::LLMPipeline pipe(model_path, "CPU");
+    ov::genai::LLMPipeline pipe(model_path, device);
 
     ov::genai::GenerationConfig config;
     config.max_new_tokens = 100;