Shortened error message and added period

Author: mcafaro

public/index.html

@@ -67,7 +67,7 @@ <h2>Generate a GitHub Actions Workflow for MATLAB</h2>
               URL or username/repository
             </label>
             <div class="invalid-feedback">
-              Please enter a valid repository URL or GitHub username and repository
+              Please enter a valid URL or username/repository.
             </div>
           </div>
           <button