Fixed bugs in yacc.html and lex.html

The Following changes has been done in the two HTML files : i.e. yacc.html && lex.html
1. Removed unnecessary CSS class.
2. Added suitable CSS classes (comparing with ywl.html)
3. Compared layout with ywl.html, and corrected it.

A change has been done in ywl.html (everything else, left untouched) i.e. I have created an empty list tag,  to ensure line break b/w "Download button and Main Title".