Update the docstring of the iiasa-connection credentials to ixmp4

[danielhuppmann]

Description of PR

@phackstock noticed that the iiasa.Connection docstring was not updated to reflect the new ixmp4-login method.

[codecov[bot]]

Codecov Report

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

Comparison is base (f4e8cbc) 94.8% compared to head (a1866de) 94.8%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #810 +/- ## ===================================== Coverage 94.8% 94.8% ===================================== Files 62 62 Lines 6042 6042 ===================================== Hits 5729 5729 Misses 313 313 ```

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