UI and text improvements for the GitHub social connection screen

I've noticed a few issues with the GitHub social connect step.

After logging in with my GitHub account, I arrive at a screen where I am asked to connect my WordPress and GitHub accounts:

- "Github" is spelled with a lowercase 'h' three times, whereas it should be uppercase: "GitHub"
- We could rewrite the main paragraph to be more concise: "Connect your WordPress.com and GitHub accounts to log in with GitHub."
- At this point in the process, asking the user to recover their password is unnecessary.
- The last paragraph and the Connect button are super wide.
- Less importantly, we could consider using a different image that displays both logos together, similar to our approach with Jetpack and WordPress in other places.

Start by locating the GitHub social connect step shown in the attached screenshot and review its copy, password-recovery prompt, layout, and image. Done means the GitHub spelling and concise paragraph are used, the unnecessary recovery prompt is removed, the final paragraph and Connect button are narrower, and the screen has an appropriate logo image.

Written by the indexing model from the issue text.