qt: Rename slot to updateDisplayUnit()
Summary:
This commit does not change behavior.
This is a backport of core#17597 [2/2]
https://github.com/bitcoin/bitcoin/pull/17597/commits/73529f0859c060087dd41e9e176fd4198d0f385f
Test Plan: ninja
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9923