Fixed a Typo.
From GitHub PR: https://github.com/Bitcoin-ABC/bitcoin-abc/pull/420
I didn't test, but given that the package is called libevent-dev, it seems obvious libevent-dec is a typo.
If someone has a Windows machine to test the instructions on, that would be helpful.
Reviewers: Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D8735