update: README example version number (#225)

Author: Velfi

README.md

@@ -16,7 +16,7 @@ The SDK provides one crate per AWS service. You must add [Tokio](https://crates.
 ```toml
 [dependencies]
 aws-config = { git = "https://github.com/awslabs/aws-sdk-rust", tag = "v0.0.18-alpha", package = "aws-config" }
-aws-sdk-dynamodb = { git = "https://github.com/awslabs/aws-sdk-rust", tag = "v0.0.17-alpha", package = "aws-sdk-dynamodb" }
+aws-sdk-dynamodb = { git = "https://github.com/awslabs/aws-sdk-rust", tag = "v0.0.18-alpha", package = "aws-sdk-dynamodb" }
 tokio = { version = "1", features = ["full"] }
 ```