fix comment

deploy/cpp_infer/tools/build.sh

-OPENCV_DIR=/paddle/Paddle/opencv-3.4.7/opencv3
-LIB_DIR=your inference dir
-CUDA_LIB_DIR=/usr/local/cuda/lib64
-CUDNN_LIB_DIR=/usr/lib/x86_64-linux-gnu
-TENSORRT_DIR=your trt dir
+OPENCV_DIR=your_opencv_dir
+LIB_DIR=your_paddle_inference_dir
+CUDA_LIB_DIR=your_cuda_lib_dir
+CUDNN_LIB_DIR=your_cudnn_lib_dir
 
 BUILD_DIR=build
 rm -rf ${BUILD_DIR}