cosmetics nginx sample config (#3385)

- fix typo
- fix spaces
- full sentence

Not tests required since no functional change happens

### Pull Request Checklist

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* [x] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [x] Pull request includes a [sign off below using a legally
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_or_ I have already signed off privately

Signed-off-by: `Werner <werner@armbian.com>`

[skip CI]

docs/nginx/dendrite-sample.conf

@@ -20,8 +20,9 @@ server {
     }
 
     location /.well-known/matrix/client {
-        # If your sever_name here doesn't match your matrix homeserver URL
+        # If your server_name here doesn't match your matrix homeserver URL
         # (e.g. hostname.com as server_name and matrix.hostname.com as homeserver URL)
+        # uncomment the following line.
         # add_header Access-Control-Allow-Origin '*';
         return 200 '{ "m.homeserver": { "base_url": "https://my.hostname.com" } }';
     }