Fix typo when constructing query string

[r1chard5mith]

(I think) opts.offset should be opts.skip consistent with adjacent lines (and the docs)

Currently the ‘skip’ parameter doesn’t work as expected when used like this:

Bing.images("Ninja Turtles", {top:3,skip:2}, function(error, result, body){ console.log(body.value); }

or this:

Bing.images("Ninja Turtles", {top:3,offset:2}, function(error, result, body){ console.log(body.value); }

but this works:

Bing.images("Ninja Turtles", {top:3,skip:2,offset:2}, function(error, result, body){ console.log(body.value); }

[goferito]

Fixed in v3.2.1