service/dynamodb/expression: Fix Builder with KeyCondition example (#2618)
service/dynamodb/expression: Fix Builder with KeyCondition example (#2618)

Fixes the ExampleBuilder_WithKeyCondition example to include the
ExpressionAttributeNames member being set.

Related to aws/aws-sdk-go-v2#285
jasdel/aws-sdk-go,yxd-hde/aws-sdk-go,yxd-hde/aws-sdk-go,aws/aws-sdk-go,yxd-hde/aws-sdk-go,yxd-hde/aws-sdk-go