Clarify how to enable the debug mode in the readme

I would like to clarify how to enable the debug mode.
At first was adding a :inspector => true meta tag to my examples.
I've found the answer in this issue
https://github.com/jonleighton/poltergeist/issues/47

At least one person more made the same mistake, so maybe its useful.
Fix typo
Merge pull request #284 from cibernox/master

Clarify how to enable the debug mode in the readme