add changelog entry for release v1.3.1 (#284)

### Smithy Go module
* `transport/http`: Loosen endpoint hostname validation to allow specifying port numbers. ([#279](#279))
* `io`: Fix RingBuffer panics due to out of bounds index. ([#282](#282))

CHANGELOG.md

@@ -1,12 +1,18 @@
-# Release v1.3.0 2021-04-01
+# Release v1.3.1 (2021-04-08)
+
+### Smithy Go module
+* `transport/http`: Loosen endpoint hostname validation to allow specifying port numbers. ([#279](/~https://github.com/aws/smithy-go/pull/279))
+* `io`: Fix RingBuffer panics due to out of bounds index. ([#282](/~https://github.com/aws/smithy-go/pull/282))
+
+# Release v1.3.0 (2021-04-01)
 
 ### Smithy Go module
 * `transport/http`: Add utility to safely join string to url path, and url raw query.
 
 ### Codegen
 * Update HttpBindingProtocolGenerator to use http/transport JoinPath and JoinQuery utility.
 
-# Release v1.2.0 2021-03-12
+# Release v1.2.0 (2021-03-12)
 
 ### Smithy Go module
 * Fix support for parsing shortened year format in HTTP Date header.