Fix typo in print statement (FORWARD example)

[romanc]

Description

The sample code for showcasing FORWARD computation (in the K axis) calls mult_upward with origin=(nhalo,nhalo,1). The print statement differs in the z-value and shows origin=(nhalo,nhalo,0). This can be confusing for developers which just skim the output and don't read the code that is actually executed. The subsequent white-space change is only for consistency.

How Has This Been Tested?

I ran this notebook locally.

Checklist:

• My code follows the style guidelines of this project
  @FlorianDeconinck isn't there automatic linting for PRs?
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[FlorianDeconinck]

CI has lint via pre-commit indeed

[FlorianDeconinck]

LGTM