Fix some PODs.

- Fix to pass tests
- Change contents to be suitable for now

perlpod cannot understand `=head` that is in `=over` and `=back`. This is spec of perlpod I think.
Thus I changed like; 
https://github.com/moznion/p5-Compiler-Lexer/compare/fix;pod?expand=1#diff-0d7cea5ad666fc43115444eb30bf2147R115

Please review it!