Fixes to topics/data-types-intro.md for typos, grammar and general readability.

I read http://redis.io/topics/data-types-intro last night, and found it very helpful as I tried to understand aspects of Redis. As I read, I also noticed and corrected some grammatical errors and typos that I'd like to contribute back.
Again, space around `--` (is `--` rendered as `–` with the used markdown implementation here? nonetheless spaces make it more readable when not rendered as html anyway)