Update the plotting example using the colormap generated by pygmt.makecpt

[seisman]

Description of proposed changes

Create a colormap using pygmt.makecpt and plot an colorbar using Figure.colorbar().

Preview the changes in this PR: https://pygmt-b5cq0nokj.now.sh/tutorials/plot.html

Fixes #470.

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.