Update build-windows.md

Summary:
Fixed a Typo.
From GitHub PR: https://github.com/Bitcoin-ABC/bitcoin-abc/pull/420

Test Plan:
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