API docs: Avoid YARD warning

[olleolleolle]

Description

This PR updates the YARD docstring annotations to avoid a warning.

``` Building YARD (yri) index for ffi-yajl-2.3.1... lib/ffi_yajl/map_library_name.rb:37: [InvalidTagFormat] Invalid tag format for @return lib/ffi_yajl/map_library_name.rb:46: [InvalidTagFormat] Invalid tag format for @return lib/ffi_yajl/map_library_name.rb:66: [InvalidTagFormat] Invalid tag format for @return ```

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have read the CONTRIBUTING document. (PS: It's blank!)
• [x] I have updated the documentation accordingly.
• [x] All commits have been signed-off for the Developer Certificate of Origin.

[tas50]

Thanks @olleolleolle