Fix message error on README

[speeddragon]

I encountered an error while using the code example in the README.

** (ArgumentError) construction of binary failed: segment 1 of type 'binary': expected a binary but got: [~c"{", [[34, "title", 34], 58, 34, "teste", 34], [[44, [34, "body", 34], 58, 34, "", 34]], ~c"}"]
    (web_push_ex 0.1.0) lib/web_push.ex:145: WebPushEx.encrypt_payload/3
    (web_push_ex 0.1.0) lib/web_push.ex:27: WebPushEx.request/3

Added to_string() because :json.encode(message) isn't the same as Jason.encode!(message), since one generates a list, the other a binary.

Feel free to change to another example than the one I recommend.

[kenichi]

🤦🏽 Thanks for this! It is done correctly in the JOSE wrapper, but I missed this occurrence in the readme.