fix typo in ch3.md

from "you should otherwise be kept private" to "you should otherwise keep private"
fixed typo

from 'ight have expected' to 'might have expected'
Fix typo

"observale" => "observable"
Corrected one typo(changed 'is has' to 'has' on line 134

There was a typo 'the first of which (perhaps called `arg1`) is has an LHS reference' on line 134 on in ch1.md. I have changed it to 'the first of which (perhaps called `arg1`) has an LHS reference'.
Correct small omission in scope examples
could be a typo on line 88 

Line 88 says, "so he can access the variable a just as we'd see before". If we'd means we had, then we need to change the verb to 'seen'. 'before' kind of implies that it can be 'we had seen before'. I have made the change in the pull request.
changed some section titles in chapter 2
added chapter 3: 'Objects'
clarified titles
typo 'the' repeated two times 

'the' word is repeated. I have fixed the typo.