Fix secrets reference syntax in README (actions#68)

packages/core/README.md

@@ -20,7 +20,7 @@ core.setOutput('outputKey', 'outputVal');
 
 #### Exporting variables/secrets
 
-You can also export variables and secrets for future steps. Variables get set in the environment automatically, while secrets must be scoped into the environment from a workflow using `{{ secret.FOO }}`. Secrets will also be masked from the logs:
+You can also export variables and secrets for future steps. Variables get set in the environment automatically, while secrets must be scoped into the environment from a workflow using `${{ secrets.FOO }}`. Secrets will also be masked from the logs:
 
 ```
 const core = require('@actions/core');