Fix CORS config verification

In the documentation about CORS config, there is a section about verification (https://doc.nuxeo.com/nxdoc/cross-origin-resource-sharing-cors/#verifying-that-the-contribution-is-taken-into-account )which states that:

Simulate preflight request

curl --verbose -H "Origin: http://www.nuxeo.com" -H "Access-Control-Request-Method: POST" -H "Access-Control-Request-Headers: X-Requested-With" -X OPTIONS http://NUXEO_SERVER/nuxeo/site/foobar/upload

With the default configuration, preflight's response looks like this:

Default response

< HTTP/1.1 200 OK
[...]

while the response with the default configuration is actually

HTTP/1.1 401
[...]

To get an HTTP/1.1 200, you have to use the request:

curl --verbose -H "Origin: http://www.nuxeo.com" -H "Access-Control-Request-Method: POST" -H "Access-Control-Request-Headers: X-Requested-With" -X OPTIONS http://NUXEO_SERVER/nuxeo/

with the default configuration

However, when a CORS configuration is in place (e.g. the "foobar" configuration given later in the documentation) and this custom configuration is taken into account correctly, the command

curl --verbose -H "Origin: http://www.nuxeo.com" -H "Access-Control-Request-Method: POST" -H "Access-Control-Request-Headers: X-Requested-With" -X OPTIONS http://NUXEO_SERVER/nuxeo/site/foobar/upload

actually returns

< HTTP/1.1 200 OK
[...]

This might seem a bit technical or overkill, but this information helps defining and verifying that a custom CORS configuration is indeed taken into account.

The current documentation seems misleading when looking whether a custom CORS configuration is or not taken into account.