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'.
Merge pull request #17 from varun06/master

Corrected one typo(changed 'is has' to 'has' on line 134