docs(guide/Developer Guide): Fixed formatting with subtitles

[tupini07]

On the main page of the developer guide:

The "Books" and "Videos" subtitles had no space between text and the '#' so it didn't render as a subtitle in the web page.

It should be:

Videos

Instead of:

Videos

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[tupini07]

I signed it!

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CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!