gistfile1.txt

input
-----
emacs - records
vim - columns
wow
windowd and linux
windows - records
macs

output
------

emacs   records
vim   columns
wow
windowd and linux
windows   records
macs


Or if you must tread your yml as simply a file then
----------------
data = File.open('test.yml','r+').readlines()
output_data = data.map do |line|
  if line.include?('- columns') or line.include?('- records')
    line.gsub(/-/, ' ')
  else
    line
  end
end

File.open('test.yml','w') do |fl|
  fl.write(output_data)
end