gh-125225: Fix column misalignment in help('topics') output (gh-125226)

The 'help("topics")' output was misaligned due to "ASSIGNMENTEXPRESSIONS"
exceeding the implicit maximum default column width of 19 characters.

Reduced the number of columns from 4 to 3 in the listtopics()
function to allow more space for longer topic names.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>