[ruby/irb] Fix step's argument

cols.size was calling Integer#size, which returns 8.

Fixing a bug of https://github.com/ruby/irb/pull/209

https://github.com/ruby/irb/commit/c93ae4be71

lib/irb/cmd/ls.rb

@@ -64,7 +64,7 @@ module IRB
 
         def col_widths(strs, cols:)
           cols.times.map do |col|
-            (col...strs.size).step(cols.size - 1).map do |i|
+            (col...strs.size).step(cols).map do |i|
               strs[i].length
             end.max
           end