Fix documents for `mruby-io`
MODIFY
mrbgems/mruby-pack/README.md
mrbgems/mruby-pack/README.md
@@ -8,7 +8,7 @@ mruby-pack provides `Array#pack` and `String#unpack` for mruby.
Add the line below into your build configuration:
```
- conf.gem :github => 'iij/mruby-pack'
+ conf.gem :core => 'mruby-pack'
```
There is no dependency on other mrbgems.

Fix documents for `mruby-io`