Fix API.md typo 

Line 1670: Removes the extra "is" from the sentence 

Register a node-style callback on this promise. When this promise is is either fulfilled or rejected, the node callback will be called back with the node.js convention where error reason is the first argument and success value is the second argument. The error argument will be null in case of success.
Merge pull request #289 from codeNgJon/patch-1

Fix API.md typo