Update CUBED_CONFIG path in example README.md (#530)

While testing cubed, I was experiencing a lot of new-user environment issues. One issue was that I'd setup `CUBED_CONFIG` to point to a directory rather than a file path, as is shown in this example. However, I was hacking on a problem and had several `yaml` files that I was testing on. Depending on my naming convention, different files might get picked up as my `CUBED_CONFIG`. A better practice is to explicitly name the file in the README example.

examples/README.md

@@ -37,14 +37,14 @@ The `add-asarray.py` script is a small example that adds two small 4x4 arrays to
 Export `CUBED_CONFIG` as described in the set up instructions, then run the script. This is for running on the local machine using the `processes` executor:
 
 ```shell
-export CUBED_CONFIG=$(pwd)/processes
+export CUBED_CONFIG=$(pwd)/processes/cubed.yaml
 python add-asarray.py
 ```
 
 This is for Lithops on AWS:
 
 ```shell
-export CUBED_CONFIG=$(pwd)/lithops/aws
+export CUBED_CONFIG=$(pwd)/lithops/aws/cubed.yaml
 python add-asarray.py
 ```