[BUG]Typo in readme for MacOS

[pauleaster]

Description

Change readme line:
On OSX: do export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH:$AF_PATH/lib64
To:
On OSX: do export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH:$AF_PATH/lib

After installing arrayfire 3.7.2, the installation directory was /opt/arrayfire and the library files were in the ./lib subdirectory.

[9prady9]

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