Fix the error message when IPython is not available

[seisman]

Description of proposed changes

When IPython is not available, fig.show() reports an error:

Cannot find IPython. Make sure you have it installed or use 'external=True' to open in an external viewer.

The error message is out-of-date. Instead, it should report:

Cannot find IPython. Make sure you have it installed or use 'method="external"' to open in an external viewer.

The issue was originally reported at https://forum.generic-mapping-tools.org/t/building-gmt-on-freebsd-12-1-release/967/20.

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.

Notes

• You can write /format in the first line of a comment to lint the code automatically