Fixed some broken links in docs (coding conventions)

Pulled from http://cibonfire.com/docs/guides/coding_conventions.html

I'm thinking about updating it to reflect
https://github.com/php-fig/fig-standards
With the obvious caveats that:
- The CodeIgniter source must reflect their coding conventions
(http://ellislab.com/codeigniter/user-guide/general/styleguide.html)
- We have to make concessions for interoperability with CodeIgniter
- Updating the entire codebase to match the updated conventions is not
going to happen overnight