Fix the link to API reference

[zhanghongce]

Describe the bug
The link to the API reference in README.md is broken after repo migration, namely the link https://bo-yuan-huang.github.io/ILAng-Doc/doxygen-output-html/namespaceilang.html is no longer valid.

To Reproduce
Steps to reproduce the behavior:

1. Click the docs:doxygen badge in README.md

Discussion: ILAng-Doc is no longer hosted under Bo-Yuan's name. However, I don't know if the university has a corresponding github.io link for use.

[yuex1994]

Thanks for noticing this -- #212 should fix this, but it has not been merged yet.

[zhanghongce]

Thanks, Yue!