spell error fix

ISSUE=4591641

git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1420 1ad973e4-5ce8-4261-8a94-b56d1f490c56

doc/build/build_from_source.md

@@ -70,7 +70,7 @@ sudo chmod a+r /usr/local/cuda/include/cudnn.h /usr/local/cuda/lib64/libcudnn*
 Then you need to set LD\_LIBRARY\_PATH, CUDA\_HOME and PATH environment variables in ~/.bashrc.
 
 ```bash
-export LD_LIBARAY_PATH=/usr/local/cuda/lib64:$LD_LIBRARY_PATH
+export LD_LIBRARY_PATH=/usr/local/cuda/lib64:$LD_LIBRARY_PATH
 export CUDA_HOME=/usr/local/cuda
 export PATH=/usr/local/cuda/bin:$PATH
 ```