Fix the BRT vignette

[tpoisot]

What the pull request does

The calculation of the adjacency matrix in the BRT vignette was wrong, in a way that gave the right result, but it was still wrong, which is generally regarded as "oof". The PR fixes this.

Type of change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• 📖 This change requires a documentation update

Checklist

• The Project.toml field version has been updated to v0.8.1

[codecov]

Codecov Report

Merging #128 (e51e606) into main (eb7156d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #128   +/-   ##
=======================================
  Coverage   58.20%   58.20%           
=======================================
  Files          35       35           
  Lines        1024     1024           
=======================================
  Hits          596      596           
  Misses        428      428           

Flag	Coverage Δ
unittests	58.20% <ø> (ø)

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[tpoisot]

remove whitespace