mruby-io: add parentheses to to_enum call

added parentheses to to_enum call where the return value is used,
explicitly specifying :each for readability.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-io/mrblib/io.rb

@@ -210,7 +210,7 @@ class IO
   #
   # 15.2.20.5.3
   def each(&block)
-    return to_enum unless block
+    return to_enum(:each) unless block
 
     while line = self.gets
       block.call(line)