Fix discription of mruby-rational and mruby-complex.

Author: mimaki

_posts/2019-11-10-mruby-2.1.0-released.markdown

@@ -37,9 +37,9 @@ The [mruby 2.1.0](https://github.com/mruby/mruby/releases/tag/2.1.0) has been en
 ### New Libraries
 
 - Add `mrbgems/mruby-rational`:  
-[dyama/mruby-rational](https://github.com/dyama/mruby-rational) has been included into the core library, and rational numbers like `1/3` can be handled.
+`Rational` class has been included into the core library, and rational numbers like `1/3` can be handled.
 - Add `mrbgems/mruby-complex`:  
-[pbosetti/mruby-complex](https://github.com/pbosetti/mruby-complex) has been included into the core library to handle complex numbers.
+`Complex` class has been included into the core library to handle complex numbers.
 
 ### New C APIs