Rename HelloWorld to HelloWorld.md

Dear Jascareer,

I am one of your Coursera mates and I am evaluating your project. Everything is OK, but you forgot to add the file extension .md to HelloWorld. I am going to give you this point as right but accept this pull request and understand that the file is not understood as markdown file if you don´t put the .md extension.

Saludos

Ikn00s