Corrected command to call uvicorn server

[JPBergsma]

Corrected the command to call the uvicorn server in setting_up_an_api.md

[codecov]

Codecov Report

Merging #937 (9b1b939) into master (11477de) will not change coverage.
The diff coverage is n/a.

❗ Current head 9b1b939 differs from pull request most recent head 21d76e2. Consider uploading reports for the commit 21d76e2 to get more accurate results

@@           Coverage Diff           @@
##           master     #937   +/-   ##
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  Coverage   92.78%   92.78%           
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  Files          67       67           
  Lines        3786     3786           
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  Hits         3513     3513           
  Misses        273      273           

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project	92.78% <ø> (ø)
validator	92.78% <ø> (ø)

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[ml-evs]

This also looks good, thanks for trying out the guide @JPBergsma! I'll set auto-merge after the tests pass