doc/mruby3.2.md: update entries for new gems
# mrbgems
-## mruby-errno
-
-- `mruby-errno` gem is now bundled.
-
## mruby-class-ext
- Add `Class#subclasses` method.
## New bundled gems
-- mruby-set
-- mruby-dir
+- mruby-errno from [https://github.com/iij/mruby-errno.git]
+- mruby-set from [https://github.com/yui-knk/mruby-set.git]
+- mruby-dir from [https://github.com/iij/mruby-dir.git]
# CVEs
