Set "frame" parameter for the first figure of the gallery example "Timestamp"

[yvonnefroehlich]

Description of proposed changes

This PR aims to set the frame parameter of pygmt.Figure.basemap for the first figure of the gallery example Timestamp.

Related to the changes in PR #2417 to fix issue #2356.

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

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

[weiji14]

Kimda unrelated, but at L13, did we want to use the first or second figure for the thumbnail in the gallery? The second one shows the 'Powered by PyGMT'.

[seisman]

Sounds a good idea to use the second one as the thumbnail.

[yvonnefroehlich]

From my side, using the second figure as the thumbnail is fine. I changed this in commit 77a0ae3.
Hope @michaelgrund also agrees, since he added this example 🙂.