Update Carthage instructions in README.md
Update Carthage instructions in README.md

The Carthage instructions had some extra back ticks that broke the Markdown rendering of the Carthage URL.
Narsail/SwiftyBeaver,SwiftyBeaver/SwiftyBeaver,SwiftyBeaver/SwiftyBeaver,SwiftyBeaver/SwiftyBeaver,Narsail/SwiftyBeaver