service/dynamodb: Fix package doc typo (#2665)

Fixes `provides has` -> `provides` typo in the package's doc for the (un)marshaling helpers.

service/dynamodb/doc_custom.go

@@ -3,7 +3,7 @@ AttributeValue Marshaling and Unmarshaling Helpers
 
 Utility helpers to marshal and unmarshal AttributeValue to and
 from Go types can be found in the dynamodbattribute sub package. This package
-provides has specialized functions for the common ways of working with
+provides specialized functions for the common ways of working with
 AttributeValues. Such as map[string]*AttributeValue, []*AttributeValue, and
 directly with *AttributeValue. This is helpful for marshaling Go types for API
 operations such as PutItem, and unmarshaling Query and Scan APIs' responses.