Add Greengrass model fix.
### SDK Enhancements
### SDK Bugs
+* Fix greengrass service model being duplicated with different casing. ([#1541](https://github.com/aws/aws-sdk-go/pull/1541))
+ * Fixes [#1540](https://github.com/aws/aws-sdk-go/issues/1540)
+ * Fixes [#1539](https://github.com/aws/aws-sdk-go/issues/1539)
