doc(codebuild): Fix Project example with GitHubSource (#1834)

`SecretParameter` was being used incorrectly and it did not pass typechecking.

The type error was:

```
Type 'SecretParameter' is missing the following properties from type 'Secret': resolve, toJSON, toList
```

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

@@ -92,7 +92,7 @@ const gitHubSource = new codebuild.GitHubSource({
   repo: 'aws-cdk',
   oauthToken: new cdk.SecretParameter(this, 'GitHubOAuthToken', {
     ssmParameter: 'my-github-token',
-  }),
+  }).value,
   webhook: true, // optional, default: false
 });
 ```