Update README.md to fix a small typo
Update README.md to fix a small typo

Change "an" to "a" to make it grammatically correct.

Before: "Perk is an well documented set of tools for building node web applications."
Proposed: "Perk is a well documented set of tools for building node web applications."
hammour/perk,hammour/perk,alarner/perk,alarner/perk