doc: fix small typo in readme

I am using this package to great success.
Just wanted to say thanks for this work guys.
Here's a small typo fix.

PR-URL: #26
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

README.md

@@ -73,7 +73,7 @@ Because of the usage of ES6 semantics, this module is supported only on `node v4
 
 
 ### **APIs**
-***stringifyAsnc***:
+***stringifyAsync***:
 
 stringifyAsync(value[, replacer][, space][, intensity], callback)