Fix file extensions of links in lexer_tutorial/index.md
This part is about controling the inner workings of LALRPOP's built-in lexer generator and using your own hand written parser.
-- [LALRPOP's lexer generator](lexer_tutorial/001_lexer_gen.md)
-- [Writing a custom lexer](lexer_tutorial/002_writing_custom_lexer.md)
-- [Using tokens with references](lexer_tutorial/003_token_references.md)
+- [LALRPOP's lexer generator](lexer_tutorial/001_lexer_gen.html)
+- [Writing a custom lexer](lexer_tutorial/002_writing_custom_lexer.html)
+- [Using tokens with references](lexer_tutorial/003_token_references.html)
