Update progress migration URL
Update progress migration URL

The URL value for the "Progress Migration" link referenced a nonexistent page (http://bluebirdjs.com/docs/API.md#progression-migration).
justsml/bluebird,justsml/bluebird,petkaantonov/bluebird,BridgeAR/bluebird,BridgeAR/bluebird,justsml/bluebird,petkaantonov/bluebird,HBOCodeLabs/bluebird,petkaantonov/bluebird,HBOCodeLabs/bluebird,BridgeAR/bluebird,HBOCodeLabs/bluebird