docs: update deploy-hydra-example.mdx

[andreadistefano]

Add server_name directive also in the listen 443 server block to deal with multiple virtual hosts.

I followed the guide on a local server I had lying around which already had nginx installed with some virtual hosts. (I know I should run it on a dedicated server but this is just a test environment).

I couldn't figure out why the provided nginx configuration was not working, until I realized that I needed to add the server_name directive also in the listen 443 server block, or nginx would serve content from a different virtual host.

I think the directive doesn't have any impact if the virtual host for Ory Hydra is the only one on the machine, but it's necessary if someone has other virtual hosts already configured.

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