Fix a bug in the "frames" tutorials

[core-man]

Description of proposed changes
In https://www.pygmt.org/latest/tutorials/frames.html#axis-labels:

fig = pygmt.Figure()
fig.basemap(
    region=[0, 10, 0, 20],
    projection="X10c/8c",
    # Plot axis, tick marks, and axis labels on the west/left and south/bottom axes
    # Plot axis and tick marks on the north/top and east/right axes
    frame=["WSne", "x+lx-axis", "y+ly-axis"],
)
fig.show()

We don't have axis and tick marks on the left and right axes. See the output:

We should add af to the frame to plot set the frame automatically.

frame=["af", "WSne", "x+lx-axis", "y+ly-axis"],

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.

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[willschlitzer]

Looks good.