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

Change `objectForKeyedSubscript:` of NSArray from simply range make to a ruby-like range literals. It make little sense by using a ruby syntax of range (..) but doing something like making NSRange with location and length. It might be a confusing, especially this syntax is designed to simulate some ruby syntax.

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.

Please be caution 001ee18 will broke the `objectForKeyedSubscript:` api, but I believe it worth to change it earlier. bfbb414 is just fix a mistake in readme file.