Alterar s para th RUBY

print "Type the text with s:"
user_input = gets.chomp                 #gets method the object string that capture text user, chomp clean the line
user_input.downcase!                    #downcase method caixa baixa letter

if user_input.include? "s"              #include if contains letter s return true
    user_input.gsub!(/s/, "th")         #gsub troca o s pelo th
else 
    print "Nothing to do here!"
end

puts "Your string is: #{user_input}"    # insert string  and variable with type users