fix(aws-codecommit): typo in README (#780)

Removal of extra bracket.

packages/@aws-cdk/aws-codecommit/README.md

@@ -40,7 +40,7 @@ const pipeline = new codepipeline.Pipeline(this, 'MyPipeline', {
 });
 const sourceStage = new codepipeline.Stage(this, 'Source', {
     pipeline,
-}));
+});
 const sourceAction = new codecommit.PipelineSourceAction(this, 'CodeCommit', {
     stage: sourceStage,
     artifactName: 'SourceOutput', // name can be arbitrary