Remove try from asynchronous add storage example

Author: iby

README.md

@@ -524,7 +524,7 @@ if you do so, any model mismatch will be thrown as an error.
 
 In general though, if migrations are expected the asynchronous variant `addStorage(_:completion:)` method is recommended instead:
 ```swift
-let migrationProgress: Progress? = try dataStack.addStorage(
+let migrationProgress: Progress? = dataStack.addStorage(
     SQLiteStore(
         fileName: "MyStore.sqlite",
         configuration: "Config2"