Update about/index.html

See changes in README.md of mruby/mruby: https://github.com/mruby/mruby/commit/77bf470cc5d3a75e86f311bf119bb8effab207c3 and https://github.com/mruby/mruby/commit/69ec2547cee129e3f3219f52b87183a6d22fd969.

Is it correct that this the source file (not generated from markdown)?