docs(guide/Conceptual Overview): add a hyphen for clarity

Minor change, but the heading for "View independent business logic..."
would be more clear if it had a hyphen, "View-independent".
Perhaps it's because I'm new to javascript and its terminology,
but I read "View" as a verb rather than a noun on the first pass and
had to read on a bit to understand that it was, instead,
referring to The View. If you go just by grammar rules,
making "view independent" into the compound adjective,
"view-independent", makes it clear that it is modifying "business logic".
(http://www.grammarbook.com/punctuation/hyphens.asp - see Rule 1).