message: fix wrong var name
diff: @@ -6,7 +6,7 @@ FlexFlow is a deep learning framework that accelerates distributed DNN training
  * [CUDNN](https://developer.nvidia.com/cudnn) is used to perform low-level operations.
  Download CUDNN and install it locally or at the system level.
  ```
  -export CUDNN=/path/to/cudnn
  +export CUDNN_HOME=/path/to/cudnn
  ```
  * [Legion](http://legion.stanford.edu) is the underlying runtime FlexFlow built on.
  