Update add my blog instructions

The add my blog modal window should be updated since it's currently asking folks to open a bugzilla bugs:

If you would like your blog added to the feed, please have a look at our guidelines and then open a request in the Eclipse Bugzilla with your feed specific information.

It should say:

Want to become a Planet Eclipse author? To add your blog to the feed, please have a look at our guidelines and then open a GitHub issue or create a pull request with your feed-specific information.