doc: Update macOS cross compilation dependencies for Focal

Summary:

The libtinfo5 package is required on Ubuntu Focal for macOS cross
compilation.

Backport of core PR19547.

Test Plan: Read the doc.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7757