making ex20 more ruby-ish

ex20.rb

@@ -1,7 +1,7 @@
-input_file = ARGV[0]
+input_file = ARGV.first
 
 def print_all(f)
-  puts f.read()
+  puts f.read
 end
 
 def rewind(f)
@@ -9,7 +9,7 @@ def rewind(f)
 end
 
 def print_a_line(line_count, f)
-  puts "#{line_count} #{f.readline()}"
+  puts "#{line_count} #{f.readline}"
 end
 
 current_file = File.open(input_file)
@@ -28,8 +28,8 @@ puts "Let's print three lines:"
 current_line = 1
 print_a_line(current_line, current_file)
 
-current_line = current_line + 1
+current_line += 1
 print_a_line(current_line, current_file)
 
-current_line = current_line + 1
+current_line += 1
 print_a_line(current_line, current_file)