[8.15] Update 'logs flowing' screenshots in Nginx tutorials (backport #1247)

[mergify]

This updates the screenshot to confirm that logs are flowing, in the two Nginx tutorials. I used the "expected" screenshot from the parent issue because it's close enough, and recreating the original by running through the two tutorials would involve too much time.

Closes: #1106

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This is an automatic backport of pull request #1247 done by Mergify.

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