That repo seems to have been created less than an hour ago.
Give Google a little time to parse the new repos, and it should show up.

Note that it is referenced by GitHub search already:

https://github.com/search?utf8=%E2%9C%93&q=RoboSpice-Extn-Retrofit-Tutorial

As the OP used the Webmaster Tools to submit its URL, the repo is now indexed in Google.

Update 2019: the section "Enabling GitHub Pages to publish your site from master or gh-pages" is now (since 2014) more precise:

To select master or gh-pages as your publishing source, you must have the branch present in your repository.
If you don't have a master or gh-pages branch, you can create them and then return to source settings to change your publishing source.

Once the correct branch is created/populated, and selected as the publishing source, the Google indexing appears to be quick.

Update 2025, as noted by James in the comments:

• The older URL submission tool has been deprecated. Now, to submit a URL (or to speed up indexing), you must use Google Search Console.
  Note that you can only verify a domain or a site that you control. For GitHub repositories, this isn’t directly possible unless you have a GitHub Pages site with a custom domain.
• You don't have to use the master branch anymore as a publication source.
  As I mentioned in 2020, you can use any branch.