Fix some typo and mistake in README

Signed-off-by: onevcat <onevcat@gmail.com>
Change NSArray's objectForKeyedSubscript: action

Change it from simply range make to a ruby-like range literals. Using '..' run from the beginning to the end inclusively, and '…' for exclude the end value. For making range from string, using 'x,y' might be a good idea.

Signed-off-by: onevcat <onevcat@gmail.com>
Merge pull request #36 from onevcat/master

Fix some typo and mistake in README. Change NSArray's objectForKeyedSubscript: action