Improve documentation in scatter3d.py gallery example

[michaelgrund]

According to the comments and review suggestions in #1006, here's an update for the documentation of the scatter3d gallery example. It now follows the documentation of the recently deployed Color points by categories gallery example.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[core-man]

Line 8:

can be visualized using a perspective 3-dimensional plot. The ``region`

We use 3D in other places (e.g., Line 5), so how about 3D instead 3-dimensional here?

[michaelgrund]

Line 8:

can be visualized using a perspective 3-dimensional plot. The ``region`

We use 3D in other places (e.g., Line 5), so how about 3D instead 3-dimensional here?

Done in d44950b.

[weiji14]

Just two small typos, otherwise looks good to me!

[core-man]

Looks great.