Update `get_next_passes` docstring

[simonrp84]

Currently, the documentation for get_next_passes is unclear. It does not specify what units should be used for the altitude. This PR updates the docstring to make clear that altitude should be in km.

• [x] Fully documented

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.99%. Comparing base (3247b15) to head (8542447). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #157 +/- ## ======================================= Coverage 87.99% 87.99% ======================================= Files 14 14 Lines 2241 2241 ======================================= Hits 1972 1972 Misses 269 269 ``` | [Flag](https://app.codecov.io/gh/pytroll/pyorbital/pull/157/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/pyorbital/pull/157/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `87.99% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[coveralls]

coverage: 87.933%. remained the same when pulling 85424473854ec9ee1c0bb5dec2cb0b0f4e781d14 on simonrp84:fix_orb_docstr into 5b4f33698773d89a2224c39c22ced639b34b039d on pytroll:main.