Fix user guide zonal notebook generate terrain function

[brendancol]

Describe the bug A clear and concise description of what the bug is.

Expected behavior A clear and concise description of what you expected to happen.

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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

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[thuydotm]

Closing as no issue with generate_terrain() was identified in zonal notebook using xarray-spatial 0.3.5.