fix broken links in README.md
The goreportcard "badge" in the README.md shows a broken image when
rendered.
cr:
@@ -250,5 +250,5 @@ The following commands can be used to pull the `VERSION` file and check the late
The Amazon SSM Agent is licensed under the Apache 2.0 License.
-[ReportCard-URL]: http://goreportcard.com/report/aws/amazon-ssm-agent
-[ReportCard-Image]: http://goreportcard.com/badge/aws/amazon-ssm-agent
+[ReportCard-URL]: https://goreportcard.com/report/aws/amazon-ssm-agent
+[ReportCard-Image]: https://goreportcard.com/badge/aws/amazon-ssm-agent
