Prevent linkification of example URL in warning message

When the submission is not a valid Git clone URL (e.g., they submitted the URL to a release page), a warning message is displayed:

Submission URL is not a Git clone URL (e.g., https://github.com/arduino-libraries/Servo).

GitHub automatically linkifies the URL in the message when it is presented as a comment in the PR thread. I feel this
linkification makes it less clear that the URL itself is intended to be the example. Wrapping it in backticks prevents
the linkification.

Author: per1234

main.go

@@ -243,7 +243,7 @@ func populateSubmission(submissionURL string, listPath *paths.Path) (submissionT
 	err = exec.Command("git", "ls-remote", normalizedURLObject.String()).Run()
 	if err != nil {
 		if _, ok := err.(*exec.ExitError); ok {
-			submission.Error = "Submission URL is not a Git clone URL (e.g., https://github.com/arduino-libraries/Servo)."
+			submission.Error = "Submission URL is not a Git clone URL (e.g., `https://github.com/arduino-libraries/Servo`)."
 			return submission, ""
 		}